Do you mean, the user's Personal Identification (PID) number? If so, it
doesn't matter if you use a number twice.
The PID is used, together with the user's Username, to generate their
Security Identifier (SID), which is used internally to identify that
user. If Tom and Dick both had PID 1234 (for example), Tom's SID will
be generated from Tom+1234, and Dick's from Dick+1234, so they will
still be different users as far as Access is concerned.
The only theoretical problem with re-using PIDs, would be if you used
the same PID for everone. Then, if someone cracked one PID, they would
in fact have cracked them all. So generally, you'd want for most of
them to be different - just like pushing your front door key a little
bit further under the mat!
HTH,
TC