You can create hierarchical groups of libraries you are not allowed to add several times the same library to a given group/subgroup. The libraries that are part of the group is not displayed on the interface, to prevent the user to select it.
However this restriction is only done template-side, the controller and module does not handle it.
To recreate the problem:
- Go to Home › Administration › Library groups
- Add a group
- Add a library
- reload the page several times
=> The same library appears several times in the group
Note that we should also replace the "Add" and "Remove" links with buttons.