diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -6512,6 +6512,7 @@
   // and patterns match.
   if (const auto *TemplateX = dyn_cast<TemplateDecl>(X)) {
     const auto *TemplateY = cast<TemplateDecl>(Y);
+    // FIXME: for C++20 concepts, check their requirements are the same.
     return isSameEntity(TemplateX->getTemplatedDecl(),
                         TemplateY->getTemplatedDecl()) &&
            isSameTemplateParameterList(TemplateX->getTemplateParameters(),
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -8655,12 +8655,21 @@
   // Check for conflicting previous declaration.
   DeclarationNameInfo NameInfo(NewDecl->getDeclName(), NameLoc);
   LookupResult Previous(*this, NameInfo, LookupOrdinaryName,
-                        ForVisibleRedeclaration);
+                        forRedeclarationInCurContext());
   LookupName(Previous, S);
 
   FilterLookupForScope(Previous, DC, S, /*ConsiderLinkage=*/false,
                        /*AllowInlineNamespace*/false);
-  if (!Previous.empty()) {
+
+  auto *Old =
+      Previous.isSingleResult() ? Previous.getAsSingle<ConceptDecl>() : nullptr;
+  bool AddToScope = true;
+  // Check for redefinition and merge with decl from other module if needed.
+  if (Old && !hasVisibleDefinition(Old) && Context.isSameEntity(NewDecl, Old)) {
+    Context.setPrimaryMergedDecl(NewDecl, Old);
+    makeMergedDefinitionVisible(Old);
+    AddToScope = false;
+  } else if (!Previous.empty()) {
     auto *Old = Previous.getRepresentativeDecl();
     Diag(NameLoc, isa<ConceptDecl>(Old) ? diag::err_redefinition :
          diag::err_redefinition_different_kind) << NewDecl->getDeclName();
@@ -8668,7 +8677,8 @@
   }
 
   ActOnDocumentableDecl(NewDecl);
-  PushOnScopeChains(NewDecl, S);
+  if (AddToScope)
+    PushOnScopeChains(NewDecl, S);
   return NewDecl;
 }
 
diff --git a/clang/test/Modules/Inputs/merge-concepts/concepts.h b/clang/test/Modules/Inputs/merge-concepts/concepts.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/merge-concepts/concepts.h
@@ -0,0 +1,6 @@
+#ifndef SAMEAS_CONCEPTS_H_
+#define SAMEAS_CONCEPTS_H_
+
+#include "same_as.h"
+
+#endif // SAMEAS_CONCEPTS_H
diff --git a/clang/test/Modules/Inputs/merge-concepts/format.h b/clang/test/Modules/Inputs/merge-concepts/format.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/merge-concepts/format.h
@@ -0,0 +1,11 @@
+#ifndef FORMAT_H
+#define FORMAT_H
+
+#include "concepts.h"
+#include "same_as.h"
+
+template <class T> void foo()
+  requires same_as<T, int>
+{}
+
+#endif // FORMAT_H
diff --git a/clang/test/Modules/Inputs/merge-concepts/modules.map b/clang/test/Modules/Inputs/merge-concepts/modules.map
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/merge-concepts/modules.map
@@ -0,0 +1,11 @@
+module "library" {
+	export *
+	module "concepts" {
+		export *
+		header "concepts.h"
+	}
+	module "format" {
+		export *
+		header "format.h"
+	}
+}
diff --git a/clang/test/Modules/Inputs/merge-concepts/same_as.h b/clang/test/Modules/Inputs/merge-concepts/same_as.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/merge-concepts/same_as.h
@@ -0,0 +1,7 @@
+#ifndef SAME_AS_H
+#define SAME_AS_H
+
+template <class T, class U>
+concept same_as = __is_same(T, U);
+
+#endif // SAME_AS_H
diff --git a/clang/test/Modules/merge-concepts.m b/clang/test/Modules/merge-concepts.m
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/merge-concepts.m
@@ -0,0 +1,9 @@
+// RUN: rm -rf %t.dir
+// RUN: mkdir %t.dir
+//
+// RUN: %clang_cc1 -xc++ -std=c++20 -fmodules -fmodule-name=library \
+// RUN:     -emit-module %S/Inputs/merge-concepts/modules.map \
+// RUN:     -o %t.dir/module.pcm
+//
+// Note that this file merely checks the module compiles. The contents of this
+// file are never read by the compiler.