Replicas were introduced to combat duping. They are exact duplicates of various event items in every way but they include the "replica" tag. They were created by EA and are perfectly legal to own/use.
While they are not rare in the sense of the word this forum would view items, some of them do spawn at low rates and are considered rare by the overall UO community. Also many of them such as gladiators collar, mythical detective boots and the jade armband to name a few are quite desirable and often sell for anywhere from 10-50m depending on the item in question