Index: clang-tidy/ClangTidyDiagnosticConsumer.cpp
===================================================================
--- clang-tidy/ClangTidyDiagnosticConsumer.cpp
+++ clang-tidy/ClangTidyDiagnosticConsumer.cpp
@@ -19,6 +19,9 @@
 #include "ClangTidyDiagnosticConsumer.h"
 #include "llvm/ADT/SmallString.h"
 
+#include <set>
+#include <tuple>
+
 namespace clang {
 namespace tidy {
 
@@ -116,11 +119,24 @@
   }
 }
 
+struct LessClangTidyError {
+  bool operator()(const ClangTidyError *LHS, const ClangTidyError *RHS) const {
+    const ClangTidyMessage &M1 = LHS->Message;
+    const ClangTidyMessage &M2 = RHS->Message;
+
+    return std::tie(M1.FilePath, M1.FileOffset, M1.Message) <
+           std::tie(M2.FilePath, M2.FileOffset, M2.Message);
+  }
+};
+
 // Flushes the internal diagnostics buffer to the ClangTidyContext.
 void ClangTidyDiagnosticConsumer::finish() {
   finalizeLastError();
-  for (const ClangTidyError &Error : Errors)
-    Context.storeError(Error);
+  std::set<const ClangTidyError*, LessClangTidyError> UniqueErrors;
+  for (const ClangTidyError &Error : Errors) {
+    if (UniqueErrors.insert(&Error).second)
+      Context.storeError(Error);
+  }
   Errors.clear();
 }
 
Index: test/clang-tidy/deduplication.cpp
===================================================================
--- /dev/null
+++ test/clang-tidy/deduplication.cpp
@@ -0,0 +1,12 @@
+// RUN: clang-tidy -checks=google-explicit-constructor %s -- | FileCheck %s
+
+template<typename T>
+class A { A(T); };
+// CHECK: :[[@LINE-1]]:11: warning: Single-argument constructors must be explicit [google-explicit-constructor]
+// CHECK-NOT: warning:
+
+
+void f() {
+  A<int> a;
+  A<double> b;
+}