If you already applied . . . and they re-post, I can imagine the following possibilities . . .
1) The posted was already paid for, and automatically reposted.
2) They could not find anyone qualified in the first posting.
3) The found someone qualified, made an offer, and then that person passed on the job - or accepted and then back out.
4) The qualifications they posted for changed.
I don't think it can hurt to re-apply, unless you are pretty much "under-qualified" . . .