diff --git patchwork/migrations/0037_python_3.py patchwork/migrations/0037_python_3.py
new file mode 100644
index 00000000..416a7045
--- /dev/null
+++ patchwork/migrations/0037_python_3.py
@@ -0,0 +1,298 @@
+import datetime
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+import patchwork.models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [('patchwork', '0036_project_commit_url_format')]
+
+    operations = [
+        migrations.AlterField(
+            model_name='check',
+            name='context',
+            field=models.SlugField(
+                default='default',
+                help_text='A label to discern check from checks of other testing systems.',  # noqa
+                max_length=255,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='check',
+            name='description',
+            field=models.TextField(
+                blank=True,
+                help_text='A brief description of the check.',
+                null=True,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='check',
+            name='state',
+            field=models.SmallIntegerField(
+                choices=[
+                    (0, 'pending'),
+                    (1, 'success'),
+                    (2, 'warning'),
+                    (3, 'fail'),
+                ],
+                default=0,
+                help_text='The state of the check.',
+            ),
+        ),
+        migrations.AlterField(
+            model_name='check',
+            name='target_url',
+            field=models.URLField(
+                blank=True,
+                help_text='The target URL to associate with this check. This should be specific to the patch.',  # noqa
+                null=True,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='comment',
+            name='submission',
+            field=models.ForeignKey(
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name='comments',
+                related_query_name='comment',
+                to='patchwork.Submission',
+            ),
+        ),
+        migrations.AlterField(
+            model_name='delegationrule',
+            name='path',
+            field=models.CharField(
+                help_text='An fnmatch-style pattern to match filenames against.',  # noqa
+                max_length=255,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='delegationrule',
+            name='priority',
+            field=models.IntegerField(
+                default=0,
+                help_text='The priority of the rule. Rules with a higher priority will override rules with lower priorities',  # noqa
+            ),
+        ),
+        migrations.AlterField(
+            model_name='delegationrule',
+            name='user',
+            field=models.ForeignKey(
+                help_text='A user to delegate the patch to.',
+                on_delete=django.db.models.deletion.CASCADE,
+                to=settings.AUTH_USER_MODEL,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='emailconfirmation',
+            name='type',
+            field=models.CharField(
+                choices=[
+                    ('userperson', 'User-Person association'),
+                    ('registration', 'Registration'),
+                    ('optout', 'Email opt-out'),
+                ],
+                max_length=20,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='event',
+            name='category',
+            field=models.CharField(
+                choices=[
+                    ('cover-created', 'Cover Letter Created'),
+                    ('patch-created', 'Patch Created'),
+                    ('patch-completed', 'Patch Completed'),
+                    ('patch-state-changed', 'Patch State Changed'),
+                    ('patch-delegated', 'Patch Delegate Changed'),
+                    ('check-created', 'Check Created'),
+                    ('series-created', 'Series Created'),
+                    ('series-completed', 'Series Completed'),
+                ],
+                db_index=True,
+                help_text='The category of the event.',
+                max_length=20,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='event',
+            name='cover',
+            field=models.ForeignKey(
+                blank=True,
+                help_text='The cover letter that this event was created for.',
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name='+',
+                to='patchwork.CoverLetter',
+            ),
+        ),
+        migrations.AlterField(
+            model_name='event',
+            name='date',
+            field=models.DateTimeField(
+                default=datetime.datetime.utcnow,
+                help_text='The time this event was created.',
+            ),
+        ),
+        migrations.AlterField(
+            model_name='event',
+            name='patch',
+            field=models.ForeignKey(
+                blank=True,
+                help_text='The patch that this event was created for.',
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name='+',
+                to='patchwork.Patch',
+            ),
+        ),
+        migrations.AlterField(
+            model_name='event',
+            name='project',
+            field=models.ForeignKey(
+                help_text='The project that the events belongs to.',
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name='+',
+                to='patchwork.Project',
+            ),
+        ),
+        migrations.AlterField(
+            model_name='event',
+            name='series',
+            field=models.ForeignKey(
+                blank=True,
+                help_text='The series that this event was created for.',
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name='+',
+                to='patchwork.Series',
+            ),
+        ),
+        migrations.AlterField(
+            model_name='patch',
+            name='number',
+            field=models.PositiveSmallIntegerField(
+                default=None,
+                help_text='The number assigned to this patch in the series',
+                null=True,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='project',
+            name='commit_url_format',
+            field=models.CharField(
+                blank=True,
+                help_text='URL format for a particular commit. {} will be replaced by the commit SHA.',  # noqa
+                max_length=2000,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='project',
+            name='list_archive_url_format',
+            field=models.CharField(
+                blank=True,
+                help_text="URL format for the list archive's Message-ID redirector. {} will be replaced by the Message-ID.",  # noqa
+                max_length=2000,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='project',
+            name='subject_match',
+            field=models.CharField(
+                blank=True,
+                default='',
+                help_text='Regex to match the subject against if only part of emails sent to the list belongs to this project. Will be used with IGNORECASE and MULTILINE flags. If rules for more projects match the first one returned from DB is chosen; empty field serves as a default for every email which has no other match.',  # noqa
+                max_length=64,
+                validators=[patchwork.models.validate_regex_compiles],
+            ),
+        ),
+        migrations.AlterField(
+            model_name='series',
+            name='name',
+            field=models.CharField(
+                blank=True,
+                help_text='An optional name to associate with the series, e.g. "John\'s PCI series".',  # noqa
+                max_length=255,
+                null=True,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='series',
+            name='total',
+            field=models.IntegerField(
+                help_text='Number of patches in series as indicated by the subject prefix(es)'  # noqa
+            ),
+        ),
+        migrations.AlterField(
+            model_name='series',
+            name='version',
+            field=models.IntegerField(
+                default=1,
+                help_text='Version of series as indicated by the subject prefix(es)',  # noqa
+            ),
+        ),
+        migrations.AlterField(
+            model_name='seriesreference',
+            name='series',
+            field=models.ForeignKey(
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name='references',
+                related_query_name='reference',
+                to='patchwork.Series',
+            ),
+        ),
+        migrations.AlterField(
+            model_name='tag',
+            name='abbrev',
+            field=models.CharField(
+                help_text='Short (one-or-two letter) abbreviation for the tag, used in table column headers',  # noqa
+                max_length=2,
+                unique=True,
+            ),
+        ),
+        migrations.AlterField(
+            model_name='tag',
+            name='pattern',
+            field=models.CharField(
+                help_text='A simple regex to match the tag in the content of a message. Will be used with MULTILINE and IGNORECASE flags. eg. ^Acked-by:',  # noqa
+                max_length=50,
+                validators=[patchwork.models.validate_regex_compiles],
+            ),
+        ),
+        migrations.AlterField(
+            model_name='tag',
+            name='show_column',
+            field=models.BooleanField(
+                default=True,
+                help_text="Show a column displaying this tag's count in the patch list view",  # noqa
+            ),
+        ),
+        migrations.AlterField(
+            model_name='userprofile',
+            name='items_per_page',
+            field=models.PositiveIntegerField(
+                default=100, help_text='Number of items to display per page'
+            ),
+        ),
+        migrations.AlterField(
+            model_name='userprofile',
+            name='send_email',
+            field=models.BooleanField(
+                default=False,
+                help_text='Selecting this option allows patchwork to send email on your behalf',  # noqa
+            ),
+        ),
+        migrations.AlterField(
+            model_name='userprofile',
+            name='show_ids',
+            field=models.BooleanField(
+                default=False,
+                help_text='Show click-to-copy patch IDs in the list view',
+            ),
+        ),
+    ]
diff --git patchwork/migrations/0038_unique_series_references.py patchwork/migrations/0038_unique_series_references.py
new file mode 100644
index 00000000..c5517ada
--- /dev/null
+++ patchwork/migrations/0038_unique_series_references.py
@@ -0,0 +1,121 @@
+from django.db import connection, migrations, models
+from django.db.models import Count
+import django.db.models.deletion
+
+
+def merge_duplicate_series(apps, schema_editor):
+    SeriesReference = apps.get_model('patchwork', 'SeriesReference')
+    Patch = apps.get_model('patchwork', 'Patch')
+
+    msgid_seriesrefs = {}
+
+    # find all SeriesReference that share a msgid but point to different series
+    # and decide which of the series is going to be the authoritative one
+    msgid_counts = (
+        SeriesReference.objects.values('msgid')
+        .annotate(count=Count('msgid'))
+        .filter(count__gt=1)
+    )
+    for msgid_count in msgid_counts:
+        msgid = msgid_count['msgid']
+        chosen_ref = None
+        for series_ref in SeriesReference.objects.filter(msgid=msgid):
+            if series_ref.series.cover_letter:
+                if chosen_ref:
+                    # I don't think this can happen, but explode if it does
+                    raise Exception(
+                        "Looks like you've got two or more series that share "
+                        "some patches but do not share a cover letter. Unable "
+                        "to auto-resolve."
+                    )
+
+                # if a series has a cover letter, that's the one we'll group
+                # everything under
+                chosen_ref = series_ref
+
+        if not chosen_ref:
+            # if none of the series have cover letters, simply use the last
+            # one (hint: this relies on Python's weird scoping for for loops
+            # where 'series_ref' is still accessible outside the loop)
+            chosen_ref = series_ref
+
+        msgid_seriesrefs[msgid] = chosen_ref
+
+    # reassign any patches referring to non-authoritative series to point to
+    # the authoritative one, and delete the other series; we do this separately
+    # to allow us a chance to raise the exception above if necessary
+    for msgid, chosen_ref in msgid_seriesrefs.items():
+        for series_ref in SeriesReference.objects.filter(msgid=msgid):
+            if series_ref == chosen_ref:
+                continue
+
+            # update the patches to point to our chosen series instead, on the
+            # assumption that all other metadata is correct
+            for patch in Patch.objects.filter(series=series_ref.series):
+                patch.series = chosen_ref.series
+                patch.save()
+
+            # delete the other series (which will delete the series ref)
+            series_ref.series.delete()
+
+
+def copy_project_field(apps, schema_editor):
+    if connection.vendor == 'postgresql':
+        schema_editor.execute(
+            """
+            UPDATE patchwork_seriesreference
+              SET project_id = patchwork_series.project_id
+            FROM patchwork_series
+              WHERE patchwork_seriesreference.series_id = patchwork_series.id
+        """
+        )
+    elif connection.vendor == 'mysql':
+        schema_editor.execute(
+            """
+            UPDATE patchwork_seriesreference, patchwork_series
+              SET patchwork_seriesreference.project_id = patchwork_series.project_id
+            WHERE patchwork_seriesreference.series_id = patchwork_series.id
+        """
+        )  # noqa
+    else:
+        SeriesReference = apps.get_model('patchwork', 'SeriesReference')
+
+        for series_ref in SeriesReference.objects.all().select_related(
+            'series'
+        ):
+            series_ref.project = series_ref.series.project
+            series_ref.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [('patchwork', '0037_python_3')]
+
+    operations = [
+        migrations.RunPython(
+            merge_duplicate_series, migrations.RunPython.noop, atomic=False
+        ),
+        migrations.AddField(
+            model_name='seriesreference',
+            name='project',
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                to='patchwork.Project',
+            ),
+        ),
+        migrations.AlterUniqueTogether(
+            name='seriesreference', unique_together={('project', 'msgid')}
+        ),
+        migrations.RunPython(
+            copy_project_field, migrations.RunPython.noop, atomic=False
+        ),
+        migrations.AlterField(
+            model_name='seriesreference',
+            name='project',
+            field=models.ForeignKey(
+                on_delete=django.db.models.deletion.CASCADE,
+                to='patchwork.Project',
+            ),
+        ),
+    ]
diff --git patchwork/models.py patchwork/models.py
index c198bc2c..e5bfecf8 100644
--- patchwork/models.py
+++ patchwork/models.py
@@ -79,7 +79,8 @@ class Project(models.Model):
     webscm_url = models.CharField(max_length=2000, blank=True)
     list_archive_url = models.CharField(max_length=2000, blank=True)
     list_archive_url_format = models.CharField(
-        max_length=2000, blank=True,
+        max_length=2000,
+        blank=True,
         help_text="URL format for the list archive's Message-ID redirector. "
         "{} will be replaced by the Message-ID.")
     commit_url_format = models.CharField(
@@ -783,6 +784,7 @@ class SeriesReference(models.Model):
     required to handle the case whereby one or more patches are
     received before the cover letter.
     """
+    project = models.ForeignKey(Project, on_delete=models.CASCADE)
     series = models.ForeignKey(Series, related_name='references',
                                related_query_name='reference',
                                on_delete=models.CASCADE)
@@ -792,7 +794,7 @@ class SeriesReference(models.Model):
         return self.msgid
 
     class Meta:
-        unique_together = [('series', 'msgid')]
+        unique_together = [('project', 'msgid')]
 
 
 class Bundle(models.Model):
diff --git patchwork/parser.py patchwork/parser.py
index 7dc66bc0..4b837ae0 100644
--- patchwork/parser.py
+++ patchwork/parser.py
@@ -16,6 +16,7 @@ import re
 
 from django.contrib.auth.models import User
 from django.db.utils import IntegrityError
+from django.db import transaction
 from django.utils import six
 
 from patchwork.models import Comment
@@ -251,16 +252,9 @@ def _find_series_by_references(project, mail):
     for ref in refs:
         try:
             return SeriesReference.objects.get(
-                msgid=ref[:255], series__project=project).series
+                msgid=ref[:255], project=project).series
         except SeriesReference.DoesNotExist:
             continue
-        except SeriesReference.MultipleObjectsReturned:
-            # FIXME: Open bug: this can happen when we're processing
-            # messages in parallel. Pick the first and log.
-            logger.error("Multiple SeriesReferences for %s in project %s!" %
-                         (ref[:255], project.name))
-            return SeriesReference.objects.filter(
-                msgid=ref[:255], series__project=project).first().series
 
 
 def _find_series_by_markers(project, mail, author):
@@ -1029,47 +1023,64 @@ def parse_mail(mail, list_id=None):
         except IntegrityError:
             raise DuplicateMailError(msgid=msgid)
 
-        # if we don't have a series marker, we will never have an existing
-        # series to match against.
-        series = None
-        if n:
-            series = find_series(project, mail, author)
+        for attempt in range(10):  # arbitrary retry count
+            try:
+                with transaction.atomic():
+                    # if we don't have a series marker, we will never have an
+                    # existing series to match against.
+                    series = None
+                    if n:
+                        series = find_series(project, mail, author)
+                    else:
+                        x = n = 1
+
+                    # We will create a new series if:
+                    # - there is no existing series to assign this patch to, or
+                    # - there is an existing series, but it already has a patch
+                    #   with this number in it
+                    if not series or Patch.objects.filter(
+                            series=series, number=x).count():
+                        series = Series.objects.create(
+                            project=project,
+                            date=date,
+                            submitter=author,
+                            version=version,
+                            total=n)
+
+                        # NOTE(stephenfin) We must save references for series.
+                        # We do this to handle the case where a later patch is
+                        # received first. Without storing references, it would
+                        # not be possible to identify the relationship between
+                        # patches as the earlier patch does not reference the
+                        # later one.
+                        for ref in refs + [msgid]:
+                            ref = ref[:255]
+                            # we don't want duplicates
+                            try:
+                                # we could have a ref to a previous series.
+                                # (For example, a series sent in reply to
+                                # another series.) That should not create a
+                                # series ref for this series, so check for the
+                                # msg-id only, not the msg-id/series pair.
+                                SeriesReference.objects.get(
+                                    msgid=ref, project=project)
+                            except SeriesReference.DoesNotExist:
+                                SeriesReference.objects.create(
+                                    msgid=ref, project=project, series=series)
+                    break
+            except IntegrityError:
+                # we lost the race so go again
+                logger.info('Conflict while saving series. This is '
+                            'probably because multiple patches belonging '
+                            'to the same series have been received at '
+                            'once. Trying again (attempt %02d/10)',
+                            attempt)
         else:
-            x = n = 1
-
-        # We will create a new series if:
-        # - there is no existing series to assign this patch to, or
-        # - there is an existing series, but it already has a patch with this
-        #   number in it
-        if not series or Patch.objects.filter(series=series, number=x).count():
-            series = Series.objects.create(
-                project=project,
-                date=date,
-                submitter=author,
-                version=version,
-                total=n)
-
-            # NOTE(stephenfin) We must save references for series. We
-            # do this to handle the case where a later patch is
-            # received first. Without storing references, it would not
-            # be possible to identify the relationship between patches
-            # as the earlier patch does not reference the later one.
-            for ref in refs + [msgid]:
-                ref = ref[:255]
-                # we don't want duplicates
-                try:
-                    # we could have a ref to a previous series. (For
-                    # example, a series sent in reply to another
-                    # series.) That should not create a series ref
-                    # for this series, so check for the msg-id only,
-                    # not the msg-id/series pair.
-                    SeriesReference.objects.get(msgid=ref,
-                                                series__project=project)
-                except SeriesReference.DoesNotExist:
-                    SeriesReference.objects.create(series=series, msgid=ref)
-                except SeriesReference.MultipleObjectsReturned:
-                    logger.error("Multiple SeriesReferences for %s"
-                                 " in project %s!" % (ref, project.name))
+            # we failed to save the series so return the series-less patch
+            logger.warning('Failed to save series. Your patch has been '
+                           'saved but this should not happen. Please '
+                           'report this as a bug and include logs')
+            return patch
 
         # add to a series if we have found one, and we have a numbered
         # patch. Don't add unnumbered patches (for example diffs sent
@@ -1107,14 +1118,9 @@ def parse_mail(mail, list_id=None):
             # message
             try:
                 series = SeriesReference.objects.get(
-                    msgid=msgid, series__project=project).series
+                    msgid=msgid, project=project).series
             except SeriesReference.DoesNotExist:
                 series = None
-            except SeriesReference.MultipleObjectsReturned:
-                logger.error("Multiple SeriesReferences for %s"
-                             " in project %s!" % (msgid, project.name))
-                series = SeriesReference.objects.filter(
-                    msgid=msgid, series__project=project).first().series
 
             if not series:
                 series = Series.objects.create(
@@ -1127,12 +1133,8 @@ def parse_mail(mail, list_id=None):
                 # we don't save the in-reply-to or references fields
                 # for a cover letter, as they can't refer to the same
                 # series
-                try:
-                    SeriesReference.objects.get_or_create(series=series,
-                                                          msgid=msgid)
-                except SeriesReference.MultipleObjectsReturned:
-                    logger.error("Multiple SeriesReferences for %s"
-                                 " in project %s!" % (msgid, project.name))
+                SeriesReference.objects.create(
+                    msgid=msgid, project=project, series=series)
 
             try:
                 cover_letter = CoverLetter.objects.create(
diff --git patchwork/tests/test_parser.py patchwork/tests/test_parser.py
index e5a2fca3..a69c64ba 100644
--- patchwork/tests/test_parser.py
+++ patchwork/tests/test_parser.py
@@ -361,17 +361,20 @@ class SeriesCorrelationTest(TestCase):
         msgids = [make_msgid()]
         project = create_project()
         series_v1 = create_series(project=project)
-        create_series_reference(msgid=msgids[0], series=series_v1)
+        create_series_reference(msgid=msgids[0], series=series_v1,
+                                project=project)
 
         # ...and three patches
         for i in range(3):
             msgids.append(make_msgid())
-            create_series_reference(msgid=msgids[-1], series=series_v1)
+            create_series_reference(msgid=msgids[-1], series=series_v1,
+                                    project=project)
 
         # now create a new series with "cover letter"
         msgids.append(make_msgid())
         series_v2 = create_series(project=project)
-        ref_v2 = create_series_reference(msgid=msgids[-1], series=series_v2)
+        ref_v2 = create_series_reference(msgid=msgids[-1], series=series_v2,
+                                         project=project)
 
         # ...and the "first patch" of this new series
         msgid = make_msgid()
diff --git patchwork/tests/utils.py patchwork/tests/utils.py
index 577183d0..4ead1781 100644
--- patchwork/tests/utils.py
+++ patchwork/tests/utils.py
@@ -282,8 +282,12 @@ def create_series(**kwargs):
 
 def create_series_reference(**kwargs):
     """Create 'SeriesReference' object."""
+    project = kwargs.pop('project', create_project())
+    series = kwargs.pop('series', create_series(project=project))
+
     values = {
-        'series': create_series() if 'series' not in kwargs else None,
+        'series': series,
+        'project': project,
         'msgid': make_msgid(),
     }
     values.update(**kwargs)
