What happens when a question is marked as duplicate then the original question is deleted.

As an example I am referring to this question 

[Can i prevent/restrict standard user from installing/unistalling software in ubuntu 12.04 desktop? [duplicate]](https://askubuntu.com/q/317738/107450)

Does the system detect such questions for review or do we just end up with questions with a dead link to now non-existent questions?