Index: clang/lib/Sema/SemaTemplate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplate.cpp
+++ clang/lib/Sema/SemaTemplate.cpp
@@ -8728,9 +8728,12 @@
   if (Previous.empty())
     return;
 
-  auto *OldConcept = dyn_cast<ConceptDecl>(Previous.getRepresentativeDecl());
+  NamedDecl *Old = Previous.getRepresentativeDecl();
+  if (auto *USD = dyn_cast<UsingShadowDecl>(Old))
+    Old = USD->getTargetDecl();
+
+  auto *OldConcept = dyn_cast<ConceptDecl>(Old);
   if (!OldConcept) {
-    auto *Old = Previous.getRepresentativeDecl();
     Diag(NewDecl->getLocation(), diag::err_redefinition_different_kind)
         << NewDecl->getDeclName();
     notePreviousDefinition(Old, NewDecl->getLocation());
@@ -8746,7 +8749,11 @@
     AddToScope = false;
     return;
   }
-  if (hasReachableDefinition(OldConcept)) {
+  if (hasReachableDefinition(OldConcept) &&
+      // When the old decl lives in the GMF and the new decl is same with the
+      // old one, we should merge them instead of complaining.
+      !(OldConcept->getOwningModule() &&
+        OldConcept->getOwningModule()->isGlobalModule())) {
     Diag(NewDecl->getLocation(), diag::err_redefinition)
         << NewDecl->getDeclName();
     notePreviousDefinition(OldConcept, NewDecl->getLocation());
@@ -8758,7 +8765,7 @@
     //        Other decls (e.g. namespaces) also have this shortcoming.
     return;
   }
-  Context.setPrimaryMergedDecl(NewDecl, OldConcept);
+  Context.setPrimaryMergedDecl(NewDecl, OldConcept->getCanonicalDecl());
 }
 
 /// \brief Strips various properties off an implicit instantiation
Index: clang/test/Modules/merge-concepts-in-GMF.cppm
===================================================================
--- /dev/null
+++ clang/test/Modules/merge-concepts-in-GMF.cppm
@@ -0,0 +1,63 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface -I%t %t/A.cppm -o %t/A.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface -I%t %t/B.cppm -o %t/B.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface -fprebuilt-module-path=%t %t/Use.cpp -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface -fprebuilt-module-path=%t %t/Use2.cpp -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface -fprebuilt-module-path=%t %t/Use3.cpp -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface -fprebuilt-module-path=%t %t/Use4.cpp -verify -fsyntax-only
+
+//--- foo.h
+template <class T, class U>
+concept same_as = __is_same(T, U);
+
+//--- A.cppm
+module;
+#include "foo.h"
+export module A;
+export using ::same_as;
+
+//--- B.cppm
+module;
+#include "foo.h"
+export module B;
+export using ::same_as;
+
+//--- Use.cpp
+// expected-no-diagnostics
+import A;
+import B;
+
+template <class T> void foo()
+  requires same_as<T, int>
+{}
+
+//--- Use2.cpp
+// expected-no-diagnostics
+#include "foo.h"
+import A;
+
+template <class T> void foo()
+  requires same_as<T, int>
+{}
+
+//--- Use3.cpp
+// expected-no-diagnostics
+import A;
+#include "foo.h"
+
+template <class T> void foo()
+  requires same_as<T, int>
+{}
+
+//--- Use4.cpp
+// expected-no-diagnostics
+import A;
+import B;
+#include "foo.h"
+
+template <class T> void foo()
+  requires same_as<T, int>
+{}