You post something again, because some people missed it or because it was posted long ago and already is gone on nearly every server. You should avoid reposts as they cause very short retention times on many Usenet news servers! Reposts are usually performed by either the original poster or someone from the group responding to an inquiry of particular posts that either may be incomplete or that have partially or completely expired on that particular group. 0-day groups are often subject to reposts as the lifespan of posts are purposely shortened in order to allow another wave of posts to be made. Reposts can also be done in order to facilitate particular posts that may have been posted to another group, but not on the group that also is relevant.