c: Allow duplicate C2x standard attributes

diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index b6a7ef4c92b..d49e508ce6d 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -4977,9 +4977,6 @@  c_parser_std_attribute (c_parser *parser, bool for_tm)
 static tree
 c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
 {
-  bool seen_deprecated = false;
-  bool seen_fallthrough = false;
-  bool seen_maybe_unused = false;
   location_t loc = c_parser_peek_token (parser)->location;
   if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
     return NULL_TREE;
@@ -5005,55 +5002,8 @@  c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
       tree attribute = c_parser_std_attribute (parser, for_tm);
       if (attribute != error_mark_node)
 	{
-	  bool duplicate = false;
-	  tree name = get_attribute_name (attribute);
-	  tree ns = get_attribute_namespace (attribute);
-	  if (ns == NULL_TREE)
-	    {
-	      /* Some standard attributes may appear at most once in
-		 each attribute list.  Diagnose duplicates and remove
-		 them from the list to avoid subsequent diagnostics
-		 such as the more general one for multiple
-		 "fallthrough" attributes in the same place (including
-		 in separate attribute lists in the same attribute
-		 specifier sequence, which is not a constraint
-		 violation).  */
-	      if (is_attribute_p ("deprecated", name))
-		{
-		  if (seen_deprecated)
-		    {
-		      error ("attribute %<deprecated%> can appear at most "
-			     "once in an attribute-list");
-		      duplicate = true;
-		    }
-		  seen_deprecated = true;
-		}
-	      else if (is_attribute_p ("fallthrough", name))
-		{
-		  if (seen_fallthrough)
-		    {
-		      error ("attribute %<fallthrough%> can appear at most "
-			     "once in an attribute-list");
-		      duplicate = true;
-		    }
-		  seen_fallthrough = true;
-		}
-	      else if (is_attribute_p ("maybe_unused", name))
-		{
-		  if (seen_maybe_unused)
-		    {
-		      error ("attribute %<maybe_unused%> can appear at most "
-			     "once in an attribute-list");
-		      duplicate = true;
-		    }
-		  seen_maybe_unused = true;
-		}
-	    }
-	  if (!duplicate)
-	    {
-	      TREE_CHAIN (attribute) = attributes;
-	      attributes = attribute;
-	    }
+	  TREE_CHAIN (attribute) = attributes;
+	  attributes = attribute;
 	}
       if (c_parser_next_token_is_not (parser, CPP_COMMA))
 	break;
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c b/gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c
index f1848a20cd5..7698434cc6d 100644
--- a/gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c
+++ b/gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c
@@ -1,13 +1,11 @@ 
-/* Test C2x deprecated attribute: duplicates.  */
+/* Test C2x deprecated attribute: duplicates (allowed after N2557).  */
 /* { dg-do compile } */
 /* { dg-options "-std=c2x -pedantic-errors" } */
 
-[[deprecated, __deprecated__]] int a; /* { dg-error "can appear at most once" } */
-[[__deprecated__, deprecated("message")]] int b; /* { dg-error "can appear at most once" } */
-int c [[deprecated("message"), deprecated]]; /* { dg-error "can appear at most once" } */
-[[deprecated, deprecated]]; /* { dg-error "can appear at most once" } */
+[[deprecated, __deprecated__]] int a;
+[[__deprecated__, deprecated("message")]] int b;
+int c [[deprecated("message"), deprecated]];
+[[deprecated, deprecated]];
 /* { dg-error "ignored" "ignored" { target *-*-* } .-1 } */
 
-/* Separate attribute lists in the same attribute specifier sequence,
-   with the same attribute in them, are OK.  */
 [[deprecated]] [[deprecated]] int d [[deprecated]] [[deprecated]];
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-4.c b/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-4.c
index 75aceff9941..a6cedcd0042 100644
--- a/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-4.c
+++ b/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-4.c
@@ -1,4 +1,4 @@ 
-/* Test C2x fallthrough attribute: duplicates.  */
+/* Test C2x fallthrough attribute: duplicates (allowed after N2557).  */
 /* { dg-do compile } */
 /* { dg-options "-std=c2x -pedantic-errors" } */
 
@@ -9,12 +9,9 @@  f (int a)
     {
     case 1:
       a++;
-      [[fallthrough, __fallthrough__]]; /* { dg-error "can appear at most once" } */
+      [[fallthrough, __fallthrough__]]; /* { dg-warning "specified multiple times" } */
     case 2:
       a++;
-      /* Separate attribute lists in the same attribute specifier
-	 sequence, with the same attribute in them, are OK (but
-	 receive a warning).  */
       [[fallthrough]] [[fallthrough]]; /* { dg-warning "specified multiple times" } */
     case 3:
       a++;
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c
index 300c0dae73c..6b997aa0033 100644
--- a/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c
+++ b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c
@@ -1,13 +1,11 @@ 
-/* Test C2x maybe_unused attribute: duplicates.  */
+/* Test C2x maybe_unused attribute: duplicates (allowed after N2557).  */
 /* { dg-do compile } */
 /* { dg-options "-std=c2x -pedantic-errors" } */
 
-[[maybe_unused, __maybe_unused__]] int a; /* { dg-error "can appear at most once" } */
-[[__maybe_unused__, maybe_unused]] int b; /* { dg-error "can appear at most once" } */
-int c [[maybe_unused, maybe_unused]]; /* { dg-error "can appear at most once" } */
-[[maybe_unused, maybe_unused]]; /* { dg-error "can appear at most once" } */
+[[maybe_unused, __maybe_unused__]] int a;
+[[__maybe_unused__, maybe_unused]] int b;
+int c [[maybe_unused, maybe_unused]];
+[[maybe_unused, maybe_unused]];
 /* { dg-error "ignored" "ignored" { target *-*-* } .-1 } */
 
-/* Separate attribute lists in the same attribute specifier sequence,
-   with the same attribute in them, are OK.  */
 [[maybe_unused]] [[maybe_unused]] int d [[maybe_unused]] [[maybe_unused]];