I have a double sided silver coin with the reverse of a walking liberty half dollar. One side with the "S" mint mark and the other has the "D". It weighs 11.93g.
Welcome to TNet, zirks39
...and no I've never seen such a unique coin.
I've read before that such coins are likely to be novelty coins for magic tricks, etc. and not actual mint errors.
Perhaps there will be some more knowledgeable responses after mine... Nick
Lots of them around... both 'period' and modern and in other denominations. They're 'gimmicked' for 'bar tricks' and conjuring purposes. Here's one on Fleabay:
If you look closely, you will find a very fine line(gap) where the coins were attached. Sometimes they are cut through the middle and two even pieces are glued together. Some of the better ones are machined on a lathe where one coin is hollowed out and the other coin is shaved down to be inserted into hollowed coin. I would say that two different mintmarks is a dead give away that this is not a mint error.