Prevent duplicate webstore install requests being serviced.

chrome/common/extensions/api/webstore_private.json

 // 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.
 
 [
   {
     "namespace":"webstorePrivate",
     "nodoc": "true",
     "functions": [
       {
@@ skipping 106 matching lines @@
           },
           {
             "name": "callback",
             "type": "function",
             "description": "Called when the user has either accepted/rejected the dialog, or some error occurred (such as invalid manifest or icon image data).",
             "optional": "true",
             "parameters": [
               {
                 "name": "result",
                 "type": "string",
-                "description": "A string result code, which will be empty upon success. The possible values in the case of errors include 'unknown_error', 'user_cancelled', 'manifest_error', 'icon_error', 'invalid_id', 'permission_denied', and 'invalid_icon_url'."
+                "description": "A string result code, which will be empty upon success. The possible values in the case of errors include 'unknown_error', 'user_cancelled', 'manifest_error', 'icon_error', 'invalid_id', 'permission_denied', 'invalid_icon_url', 'signin_failed', and 'already_installed'."
               }
             ]
           }
         ]
 
       },
       {
         "name": "completeInstall",
         "description": "",
         "parameters": [
@@ skipping 97 matching lines @@
             "optional": "false",
             "parameters": [
               { "name": "is_enabled", "type": "boolean" }
             ]
           }
         ]
       }
     ]
   }
 ]