>filename is a SHA256 hash - probably salted
each IP gets a 6-char hexcode (uid) which controlled by some rule is different in every bread.
6 chars (6 bytes) in hexadecimal give 16777215 possible different codes (with 000000 reserved for tor users).
So yes, the longer a board is active, the higher the probability that uids repeat, and I have seen this as well here on /qr/.