Ideally, you should dispose of the rodent and thaw a new one.
However, I think we've all been guilty of re-freezing at some point. I know I do it on occasion. I also put them in the fridge to offer the next day. I haven't had any problems with this, but I know that it's poor husbandry and I can't with good conscience reccommend it.
The only thing I'd be concerned about is that, as I've said before, hot water accelerates decay in the mouse's innards. Eew. Re-freezing and thawing again would seem to make this more likely especially if hot water was used. However, out of a couple dozen snakes I've kept, only two have ever been really sensitive to 'too thawed' prey and failed to keep it down. So, it's basically up to you. If the snake seems to have trouble with it, obviously, you'll want to stop doing that.
I sometimes re-freeze and offer again, but if the snake doesn't take that time, I always dispose of the mouse and use a new one next time. Two re-freezes just seems too yucky to me.
Best of luck, and always listen to your snake!
--Naamah