Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td	(revision 208175)
+++ include/clang/Basic/DiagnosticSemaKinds.td	(working copy)
@@ -3393,6 +3393,10 @@
 // C++ Explicit Instantiation
 def err_explicit_instantiation_duplicate : Error<
     "duplicate explicit instantiation of %0">;
+def warn_explicit_instantiation_duplicate : ExtWarn<
+    "duplicate explicit instantiation of %0 ignored in Microsoft compatibility "
+    "mode">,
+    InGroup<Microsoft>;
 def note_previous_explicit_instantiation : Note<
     "previous explicit instantiation is here">;
 def ext_explicit_instantiation_after_specialization : Extension<
Index: lib/Sema/SemaTemplate.cpp
===================================================================
--- lib/Sema/SemaTemplate.cpp	(revision 208175)
+++ lib/Sema/SemaTemplate.cpp	(working copy)
@@ -6517,8 +6517,12 @@
       //   For a given template and a given set of template-arguments,
       //     - an explicit instantiation definition shall appear at most once
       //       in a program,
-      Diag(NewLoc, diag::err_explicit_instantiation_duplicate)
-        << PrevDecl;
+
+      // MSVCCompat: MSVC silently ignores duplicate explicit instantiations.
+      Diag(NewLoc, (getLangOpts().MSVCCompat)
+                       ? diag::warn_explicit_instantiation_duplicate
+                       : diag::err_explicit_instantiation_duplicate)
+          << PrevDecl;
       Diag(DiagLocForExplicitInstantiation(PrevDecl, PrevPointOfInstantiation),
            diag::note_previous_explicit_instantiation);
       HasNoEffect = true;
Index: test/SemaTemplate/ms-class-specialization-duplicate.cpp
===================================================================
--- test/SemaTemplate/ms-class-specialization-duplicate.cpp	(revision 0)
+++ test/SemaTemplate/ms-class-specialization-duplicate.cpp	(working copy)
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fms-compatibility -fdelayed-template-parsing -fsyntax-only -verify %s
+
+template <typename T>
+class A {
+};
+typedef int TInt;
+
+template class A<int>;  // expected-note {{previous explicit instantiation is here}}
+template class A<TInt>; // expected-warning {{duplicate explicit instantiation of 'A<int>' ignored in Microsoft compatibility mode}}