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

chromeos/printing/fake_printer_discoverer.cc

Index: chromeos/printing/fake_printer_discoverer.cc
diff --git a/chromeos/printing/fake_printer_discoverer.cc b/chromeos/printing/fake_printer_discoverer.cc
index eb5804e06e629698251500b17103c46b8ef4f089..53828d5cfcd673630b61bf4fa9a12c615272b5ed 100644
--- a/chromeos/printing/fake_printer_discoverer.cc
+++ b/chromeos/printing/fake_printer_discoverer.cc
@@ -46,7 +46,9 @@ bool FakePrinterDiscoverer::StopDiscovery() {
 }
 
 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(