Does a collision occur if two packets are sent at the same time in Slotted AHOLA?
A station is not allowed to send two packets at the same time.  Each station is only allowed to send one packet at a time.  If the station has multiple packets it wants to send, it can start sending one of them but it must queue the others and wait to send them later.

The protocol does not impose any requirements on how long a station must wait to send the other packets.  Suppose Alice has two packets to send.  Alice is required wait to send the second packet until some later point after she has finished sending the first packet.  Beyond that, it's up to her exactly when he tries to send it.  The protocol doesn't specify anything in particular.   Some stations might try to send the second packet immediately after they finish sending the first packet.  Other stations might wait a while (for whatever reason) in between.  There are no guarantees and no requirements.