fix #105923.

src/vs/workbench/contrib/notebook/browser/notebook.contribution.ts

@@ -346,7 +346,7 @@ export class NotebookContribution extends Disposable implements IWorkbenchContri
 		const existingEditors = group.editors.filter(editor => editor.resource && isEqual(editor.resource, notebookUri) && !(editor instanceof NotebookEditorInput));
 
 		if (existingEditors.length) {
-			return undefined;
+			return { override: this.editorService.openEditor(existingEditors[0]) };
 		}
 
 		const userAssociatedEditors = this.getUserAssociatedEditors(notebookUri);