Does MySql event will start if the previous event doesn't finish yet?
To quote the manual:


  If a repeating event does not terminate within its scheduling interval, the result may be multiple instances of the event executing simultaneously. If this is undesirable, you should institute a mechanism to prevent simultaneous instances. For example, you could use the GET_LOCK() function, or row or table locking. 


In other words, the new copy will run.

This could lead to more and more running, so you should worry about it.