I find that I am annoyed when someone posts a response to a Fast Friday challenge with a piece that had already been made. It doesn't matter whether I am actually planning to submit something myself, or just that someone else got in with the first posting. What bothers me is that it is not actually something made that was inspired by that challenge, however closely it seems to fit. We're not running a race to see who can post first; we are trying to delve into aspects of art that maybe not all of us are familiar with, to possibly branch out into a new area.
It may be that the current challenge is something quite familiar, fits right in with our style, whatever it may be, and all the elements fit together so that within a few days we have something new. For some challenges, we hit a complete block and can't get into it. All this is okay. And sometimes other events get in the way, and we make and post a response months later - even years later. That's fine.
But to post a previously completed project -- that is not a response to a challenge, as I see it.