I remember a discussion about this custom of saying "rabbit rabbit" on a 2006 thread. I believe that the thread was about wearing green on St. Patrick day but I'm not sure.

There's also some discussion about that custom on RE: BS: Pinch, punch, 1st. of the month.

Also, see this post in thread.cfm?threadid=11086#81573
"RE: Forbidden or unlucky words? (Mostly BS)"