Redmine

Create a project A, then a subproject A1, and check "Inherit members" on A1.

Then go to Users / user_xx / Projects and click "Add".

In Project list, select projects A, A1, and any role, then click "Add": The message "Failed to save member(s): User has already been taken" will appear.

In my opinion, if "Inherit members" is selected on subproject A1, and we select project A on the modal, then the A1 project checkbox should be disabled, but checked automatically to tell the user there is an inheritance rule.