Prevent duplicate webstore install requests being serviced.

chrome/browser/extensions/api/webstore_private/webstore_private_api.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_API_WEBSTORE_PRIVATE_WEBSTORE_PRIVATE_API_H_
 #define CHROME_BROWSER_EXTENSIONS_API_WEBSTORE_PRIVATE_WEBSTORE_PRIVATE_API_H_
 
 #include <string>
 
 #include "chrome/browser/extensions/bundle_installer.h"
@@ skipping 88 matching lines @@
     // The extension id was invalid.
     INVALID_ID,
 
     // The page does not have permission to call this function.
     PERMISSION_DENIED,
 
     // Invalid icon url.
     INVALID_ICON_URL,
 
     // Signin has failed.
-    SIGNIN_FAILED
+    SIGNIN_FAILED,
+
+    // An extension with the same extension id has already been installed.
+    ALREADY_INSTALLED,
   };
 
   BeginInstallWithManifestFunction();
 
   // WebstoreInstallHelper::Delegate:
   virtual void OnWebstoreParseSuccess(
       const std::string& id,
       const SkBitmap& icon,
       base::DictionaryValue* parsed_manifest) OVERRIDE;
   virtual void OnWebstoreParseFailure(
@@ skipping 165 matching lines @@
   // ExtensionFunction:
   virtual bool RunImpl() OVERRIDE;
 
  private:
   void OnIsLauncherCheckCompleted(bool is_enabled);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_WEBSTORE_PRIVATE_WEBSTORE_PRIVATE_API_H_