Disallow adding duplicate device ID in GUI

Adds check in valid device id to check for uniqueness

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;
                 });

gui/syncthing/device/editDeviceModalView.html

@@ -21,6 +21,7 @@
               <span translate ng-show="!editingExisting && (deviceEditor.deviceID.$valid || deviceEditor.deviceID.$pristine)">When adding a new device, keep in mind that this device must be added on the other side too.</span>
               <span translate ng-if="deviceEditor.deviceID.$error.required && deviceEditor.deviceID.$dirty">The device ID cannot be blank.</span>
               <span translate ng-if="deviceEditor.deviceID.$error.validDeviceid && deviceEditor.deviceID.$dirty">The entered device ID does not look valid. It should be a 52 or 56 character string consisting of letters and numbers, with spaces and dashes being optional.</span>
+              <span translate ng-if="deviceEditor.deviceID.$error.unique && deviceEditor.deviceID.$dirty">A device with that ID is already added.</span>
             </p>
           </div>
           <div class="form-group">