diff --git a/src/vs/workbench/api/browser/viewsContainersExtensionPoint.ts b/src/vs/workbench/api/browser/viewsContainersExtensionPoint.ts
index 5528735ff51..5fd69ca019e 100644
--- a/src/vs/workbench/api/browser/viewsContainersExtensionPoint.ts
+++ b/src/vs/workbench/api/browser/viewsContainersExtensionPoint.ts
@@ -143,56 +143,62 @@ class ViewsContainersExtensionHandler implements IWorkbenchContribution {
 	}
 
 	private registerCustomViewlet(descriptor: IUserFriendlyViewsContainerDescriptor, order: number, cssClass: string): void {
+		const viewletRegistry = Registry.as<ViewletRegistry>(ViewletExtensions.Viewlets);
 		const id = `workbench.view.extension.${descriptor.id}`;
-		const location: ViewLocation = ViewLocation.register(id);
 
-		// Register as viewlet
-		class CustomViewlet extends PersistentViewsViewlet {
-			constructor(
-				@IPartService partService: IPartService,
-				@ITelemetryService telemetryService: ITelemetryService,
-				@IWorkspaceContextService contextService: IWorkspaceContextService,
-				@IStorageService storageService: IStorageService,
-				@IWorkbenchEditorService editorService: IWorkbenchEditorService,
-				@IInstantiationService instantiationService: IInstantiationService,
-				@IContextKeyService contextKeyService: IContextKeyService,
-				@IThemeService themeService: IThemeService,
-				@IContextMenuService contextMenuService: IContextMenuService,
-				@IExtensionService extensionService: IExtensionService
-			) {
-				super(id, location, `${id}.state`, true, partService, telemetryService, storageService, instantiationService, themeService, contextService, contextKeyService, contextMenuService, extensionService);
+		if (!viewletRegistry.getViewlet(id)) {
+
+			const location: ViewLocation = ViewLocation.register(id);
+
+			// Register as viewlet
+			class CustomViewlet extends PersistentViewsViewlet {
+				constructor(
+					@IPartService partService: IPartService,
+					@ITelemetryService telemetryService: ITelemetryService,
+					@IWorkspaceContextService contextService: IWorkspaceContextService,
+					@IStorageService storageService: IStorageService,
+					@IWorkbenchEditorService editorService: IWorkbenchEditorService,
+					@IInstantiationService instantiationService: IInstantiationService,
+					@IContextKeyService contextKeyService: IContextKeyService,
+					@IThemeService themeService: IThemeService,
+					@IContextMenuService contextMenuService: IContextMenuService,
+					@IExtensionService extensionService: IExtensionService
+				) {
+					super(id, location, `${id}.state`, true, partService, telemetryService, storageService, instantiationService, themeService, contextService, contextKeyService, contextMenuService, extensionService);
+				}
 			}
-		}
-		const viewletDescriptor = new ViewletDescriptor(
-			CustomViewlet,
-			id,
-			descriptor.title,
-			cssClass,
-			order
-		);
+			const viewletDescriptor = new ViewletDescriptor(
+				CustomViewlet,
+				id,
+				descriptor.title,
+				cssClass,
+				order
+			);
 
-		Registry.as<ViewletRegistry>(ViewletExtensions.Viewlets).registerViewlet(viewletDescriptor);
+			viewletRegistry.registerViewlet(viewletDescriptor);
 
-		// Register Action to Open Viewlet
-		class OpenCustomViewletAction extends ToggleViewletAction {
-			constructor(
-				id: string, label: string,
-				@IViewletService viewletService: IViewletService,
-				@IWorkbenchEditorService editorService: IWorkbenchEditorService
-			) {
-				super(id, label, id, viewletService, editorService);
+			// Register Action to Open Viewlet
+			class OpenCustomViewletAction extends ToggleViewletAction {
+				constructor(
+					id: string, label: string,
+					@IViewletService viewletService: IViewletService,
+					@IWorkbenchEditorService editorService: IWorkbenchEditorService
+				) {
+					super(id, label, id, viewletService, editorService);
+				}
 			}
-		}
-		const registry = Registry.as<IWorkbenchActionRegistry>(ActionExtensions.WorkbenchActions);
-		registry.registerWorkbenchAction(
-			new SyncActionDescriptor(OpenCustomViewletAction, id, localize('showViewlet', "Show {0}", descriptor.title)),
-			'View: Show {0}',
-			localize('view', "View")
-		);
+			const registry = Registry.as<IWorkbenchActionRegistry>(ActionExtensions.WorkbenchActions);
+			registry.registerWorkbenchAction(
+				new SyncActionDescriptor(OpenCustomViewletAction, id, localize('showViewlet', "Show {0}", descriptor.title)),
+				'View: Show {0}',
+				localize('view', "View")
+			);
+
+			// Generate CSS to show the icon in the activity bar
+			const iconClass = `.monaco-workbench > .activitybar .monaco-action-bar .action-label.${cssClass}`;
+			createCSSRule(iconClass, `-webkit-mask: url('${descriptor.icon}') no-repeat 50% 50%`);
+		}
 
-		// Generate CSS to show the icon in the activity bar
-		const iconClass = `.monaco-workbench > .activitybar .monaco-action-bar .action-label.${cssClass}`;
-		createCSSRule(iconClass, `-webkit-mask: url('${descriptor.icon}') no-repeat 50% 50%`);
 	}
 }