Waiting list allowing multiple booking from one user

Hi, 
Waiting list is allowing more than one booking per person. Is this normal? 
We have this event that is full, and people are still registering to go on a waiting list but I saw one user booking himself twice.  It is also allowing people on the waiting list to be checked in. 
Is there any setting we can use to prevent this? 
 

Hello Henry

Is user logged in? If Yes, the system will prevent them from joining waiting list twice

If he is not logged in, the system won't know who he is, so it cannot prevent him from joining waiting list again

Tuan