diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -113,6 +113,8 @@
   This fixes Issue `Issue 54462 <https://github.com/llvm/llvm-project/issues/54462>`_.
 - Statement expressions are now disabled in default arguments in general.
   This fixes Issue `Issue 53488 <https://github.com/llvm/llvm-project/issues/53488>`_.
+- Unknown attributes with [[]] spelling now will only be warned once.
+  This fixes Issue `Issue 54817 <https://github.com/llvm/llvm-project/issues/54817>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -2633,8 +2633,12 @@
   // Forbid C++11 and C2x attributes that appear on certain syntactic locations
   // which standard permits but we don't supported yet, for example, attributes
   // appertain to decl specifiers.
+  // For the most cases we don't want to warn on unknown attributes, but left
+  // them to later diagnoses. However, for a few cases like module declarations
+  // and module import declarations, we should do it.
   void ProhibitCXX11Attributes(ParsedAttributes &Attrs, unsigned DiagID,
-                               bool DiagnoseEmptyAttrs = false);
+                               bool DiagnoseEmptyAttrs = false,
+                               bool WarnOnUnknownAttrs = false);
 
   /// Skip C++11 and C2x attributes and return the end location of the
   /// last one.
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -1658,7 +1658,8 @@
 }
 
 void Parser::ProhibitCXX11Attributes(ParsedAttributes &Attrs, unsigned DiagID,
-                                     bool DiagnoseEmptyAttrs) {
+                                     bool DiagnoseEmptyAttrs,
+                                     bool WarnOnUnknownAttrs) {
 
   if (DiagnoseEmptyAttrs && Attrs.empty() && Attrs.Range.isValid()) {
     // An attribute list has been parsed, but it was empty.
@@ -1685,10 +1686,11 @@
   for (const ParsedAttr &AL : Attrs) {
     if (!AL.isCXX11Attribute() && !AL.isC2xAttribute())
       continue;
-    if (AL.getKind() == ParsedAttr::UnknownAttribute)
-      Diag(AL.getLoc(), diag::warn_unknown_attribute_ignored)
-          << AL << AL.getRange();
-    else {
+    if (AL.getKind() == ParsedAttr::UnknownAttribute) {
+      if (WarnOnUnknownAttrs)
+        Diag(AL.getLoc(), diag::warn_unknown_attribute_ignored)
+            << AL << AL.getRange();
+    } else {
       Diag(AL.getLoc(), DiagID) << AL;
       AL.setInvalid();
     }
diff --git a/clang/lib/Parse/Parser.cpp b/clang/lib/Parse/Parser.cpp
--- a/clang/lib/Parse/Parser.cpp
+++ b/clang/lib/Parse/Parser.cpp
@@ -2377,7 +2377,9 @@
   // We don't support any module attributes yet; just parse them and diagnose.
   ParsedAttributes Attrs(AttrFactory);
   MaybeParseCXX11Attributes(Attrs);
-  ProhibitCXX11Attributes(Attrs, diag::err_attribute_not_module_attr);
+  ProhibitCXX11Attributes(Attrs, diag::err_attribute_not_module_attr,
+                          /*DiagnoseEmptyAttrs=*/false,
+                          /*WarnOnUnknownAttrs=*/true);
 
   ExpectAndConsumeSemi(diag::err_module_expected_semi);
 
@@ -2444,7 +2446,9 @@
   ParsedAttributes Attrs(AttrFactory);
   MaybeParseCXX11Attributes(Attrs);
   // We don't support any module import attributes yet.
-  ProhibitCXX11Attributes(Attrs, diag::err_attribute_not_import_attr);
+  ProhibitCXX11Attributes(Attrs, diag::err_attribute_not_import_attr,
+                          /*DiagnoseEmptyAttrs=*/false,
+                          /*WarnOnUnknownAttrs=*/true);
 
   if (PP.hadModuleLoaderFatalFailure()) {
     // With a fatal failure in the module loader, we abort parsing.
diff --git a/clang/test/SemaCXX/warn-once-on-unknown-attr.cpp b/clang/test/SemaCXX/warn-once-on-unknown-attr.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/warn-once-on-unknown-attr.cpp
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c2x -x c %s
+void foo() {
+  int [[attr]] i; // expected-warning {{unknown attribute 'attr' ignored}}
+  (void)sizeof(int [[attr]]); // expected-warning {{unknown attribute 'attr' ignored}}
+}
+
+
+void bar() {
+  [[attr]]; // expected-warning {{unknown attribute 'attr' ignored}}
+  [[attr]] int i; // expected-warning {{unknown attribute 'attr' ignored}}
+}