"collision" , are refers to the hash table entries

using open addressing also collisions can be avoided.
Linux uses chaining, where the data entry hashes are filled with linked list of indexes of the data

As mfillpot mentioned, two processes can not have same PIDs. Linux will assign a PID to a process and if the process is finished, that PID get used, once the PID limit in OS exceeds and started to assign the PID again from 2 onwards. [ start from beginning. ]