Merge branch 'pr/2628'

* pr/2628:
  Disallow adding duplicate device ID in GUI

gui/syncthing/core/validDeviceidDirective.js

@@ -15,6 +15,15 @@ angular.module('syncthing.core')
                                 ctrl.$setValidity('validDeviceid', true);
                             }
                         });
+                        //Prevents user from adding a duplicate ID
+                        var matches = scope.devices.filter(function (n) {
+                            return n.deviceID == viewValue;
+                        }).length;
+                        if (matches > 0) {
+                            ctrl.$setValidity('unique', false);
+                        } else {
+                            ctrl.$setValidity('unique', true);
+                        }
                     }
                     return viewValue;
                 });