Answer is No
Non-contention based procedure is always initiated from network in case of a handover. For this procedure, the eNB reserves a set of preamble sequence.This entire procedure is controlled by the eNB. Hence. no question of collision.
Yes contention based is possible in below scenario: If target ENb didn't give preambles to do hand over rach , then UE has to do contention based rach on target ENB.
weather it is RRCConnection request or direct NAS message as ULInfotransfer?
Does Other UE knows, whether its a Contention Based or Non-Contention based procedure ?