[CUPS] Fix the occasional crash when switching between different dialogs.

chromeos/printing/fake_printer_discoverer.cc

 // Copyright 2016 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.
 
 #include "chromeos/printing/fake_printer_discoverer.h"
 
 #include <algorithm>
 #include <iterator>
 #include <memory>
 
@@ skipping 28 matching lines @@
 
 bool FakePrinterDiscoverer::StopDiscovery() {
   discovery_running_ = false;
   for (auto observer : observers_)
     observer->OnDiscoveryStopping();
 
   return true;
 }
 
 void FakePrinterDiscoverer::AddObserver(PrinterDiscoverer::Observer* observer) {
-  observers_.push_back(observer);
+  auto found = std::find(observers_.begin(), observers_.end(), observer);
+  if (found == observers_.end())
+    observers_.push_back(observer);
 }
 
 void FakePrinterDiscoverer::RemoveObserver(
     PrinterDiscoverer::Observer* observer) {
   auto found = std::find(observers_.begin(), observers_.end(), observer);
   if (found != observers_.end())
     observers_.erase(found);
 }
 
 FakePrinterDiscoverer::FakePrinterDiscoverer()
@@ skipping 36 matching lines @@
         base::TimeDelta::FromMilliseconds(2000));
   } else {
     // We're done.  Notify observers.
     discovery_running_ = false;
     for (auto observer : observers_)
       observer->OnDiscoveryDone();
   }
 }
 
 }  // namespace chromeos