Currently it is possible to add and maintain same National ID value for 2 different dependents, though validation for National ID is enabled in Company System and Logo Settings.
Employee Central: Dependents Management
This will be incorporated in b1802 release. A validation message will get thrown when one employee has two dependents with same National ID.
ECT-86997 , KBA , LOD-SF-EC-DPD , Dependents Management , Product Enhancement