Fix a duplicate layout parent issue

gtk2_ardour/engine_dialog.cc

@@ -662,7 +662,7 @@ EngineControl::build_full_control_notebook ()
 	}
 
 	/* Prefere next available vertical slot, 1 row */
-	if (btn < row) {
+	if (btn < row && !autostart_packed) {
 		basic_packer.attach (try_autostart_button, 3, 4, btn, btn + 1, xopt, xopt);
 		btn++;
 		autostart_packed = true;