Steps to reproduce:
1) Create map.
2) Add element with Type - Map and added another Map.
3) Navigate to dashboard.
4) Create new widget - Map navigation tree.
5) Add new child element to widget. Choose Map created before. Also check-in option - "Add submaps".
We see something like that.

6) Click on Edit icon and in appeared form again check-in - "Add submaps" and click Apply.

Result:

Expected:
Submap added only once.