Index: clang-tidy/ClangTidyDiagnosticConsumer.cpp
===================================================================
--- clang-tidy/ClangTidyDiagnosticConsumer.cpp
+++ clang-tidy/ClangTidyDiagnosticConsumer.cpp
@@ -239,10 +239,11 @@
   finalizeLastError();
   std::set<const ClangTidyError*, LessClangTidyError> UniqueErrors;
   for (const ClangTidyError &Error : Errors) {
-    if (Context.getChecksFilter().isCheckEnabled(Error.CheckName) &&
-        UniqueErrors.insert(&Error).second)
-      Context.storeError(Error);
+    if (Context.getChecksFilter().isCheckEnabled(Error.CheckName))
+      UniqueErrors.insert(&Error);
   }
+  for (const ClangTidyError *Error : UniqueErrors)
+    Context.storeError(*Error);
   Errors.clear();
 }
 
Index: unittests/clang-tidy/CMakeLists.txt
===================================================================
--- unittests/clang-tidy/CMakeLists.txt
+++ unittests/clang-tidy/CMakeLists.txt
@@ -7,6 +7,7 @@
 include_directories(${CLANG_LINT_SOURCE_DIR})
 
 add_extra_unittest(ClangTidyTests
+  ClangTidyDiagnosticConsumerTest.cpp
   LLVMModuleTest.cpp
   GoogleModuleTest.cpp
   MiscModuleTest.cpp)
Index: unittests/clang-tidy/ClangTidyDiagnosticConsumerTest.cpp
===================================================================
--- /dev/null
+++ unittests/clang-tidy/ClangTidyDiagnosticConsumerTest.cpp
@@ -0,0 +1,33 @@
+#include "ClangTidy.h"
+#include "ClangTidyTest.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace tidy {
+namespace test {
+
+class TestCheck : public ClangTidyCheck {
+public:
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override {
+    Finder->addMatcher(ast_matchers::varDecl().bind("var"), this);
+  }
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override {
+    const VarDecl *Var = Result.Nodes.getNodeAs<VarDecl>("var");
+    // Add diagnostics in the wrong order.
+    diag(Var->getLocation(), "variable");
+    diag(Var->getTypeSpecStartLoc(), "type specifier");
+  }
+};
+
+TEST(ClangTidyDiagnosticConsumer, SortsErrors) {
+  SmallVector<ClangTidyError, 8> Errors;
+  runCheckOnCode<TestCheck>("int a;", Errors);
+  EXPECT_EQ(2ul, Errors.size());
+  // FIXME: Remove " []" once the check name is removed from the message text.
+  EXPECT_EQ("type specifier []", Errors[0].Message.Message);
+  EXPECT_EQ("variable []", Errors[1].Message.Message);
+}
+
+} // namespace test
+} // namespace tidy
+} // namespace clang
Index: unittests/clang-tidy/ClangTidyTest.h
===================================================================
--- unittests/clang-tidy/ClangTidyTest.h
+++ unittests/clang-tidy/ClangTidyTest.h
@@ -39,9 +39,10 @@
   ClangTidyContext *Context;
 };
 
-template <typename T> std::string runCheckOnCode(StringRef Code) {
+template <typename T>
+std::string runCheckOnCode(StringRef Code,
+                           SmallVectorImpl<ClangTidyError> &Errors) {
   T Check;
-  SmallVector<ClangTidyError, 16> Errors;
   ClangTidyContext Context(&Errors, ".*", "");
   ClangTidyDiagnosticConsumer DiagConsumer(Context);
   Check.setContext(&Context);
@@ -65,6 +66,11 @@
   return tooling::applyAllReplacements(Code, Fixes);
 }
 
+template <typename T> std::string runCheckOnCode(StringRef Code) {
+  SmallVector<ClangTidyError, 16> Errors;
+  return runCheckOnCode<T>(Code, Errors);
+}
+
 } // namespace test
 } // namespace tidy
 } // namespace clang