diff --git a/artiq/gui/applets.py b/artiq/gui/applets.py
index 09a1fe56c..be9ebd6b0 100644
--- a/artiq/gui/applets.py
+++ b/artiq/gui/applets.py
@@ -288,6 +288,7 @@ class AppletsDock(QtWidgets.QDockWidget):
         if uid is None:
             uid = next(iter(set(range(len(self.applet_uids) + 1))
                             - self.applet_uids))
+        assert uid not in self.applet_uids
         self.applet_uids.add(uid)
 
         row = self.table.rowCount()
@@ -328,7 +329,6 @@ class AppletsDock(QtWidgets.QDockWidget):
             self.applet_uids.remove(item.applet_uid)
             self.table.removeRow(row)
 
-
     async def stop(self):
         for row in range(self.table.rowCount()):
             dock = self.table.item(row, 0).applet_dock