Picked up 2 rolls of halves today, 1 coin has a double reverse. Never encountered this before. Doesn't look like it's been altered, perhaps a mint error? Not sending a picture of the coin, not really necessary.
When you say double reverse, do you mean it appears to have been stamped twice on the reverse side? If so that would be what is called a double die, which is known to happen from time to time.
Or do you mean it has no obverse side and there are eagles on both sides? Which I've never heard of happening.
If you mean it's tails on both sides it's not possible for this to happen at the mint. It's referred to as a magicians coin and they are made from 2 separate coins.
I've seen Mercury dimes done like that too, and they looked good, seems impossible to slice a coin in half and put it back together but I know some people have that ability. Nice find though, and certainly worth more than .50c face, and probably worth more than a 90% half too.