What does the bank hope to accomplish by claiming a coin was already spent? It can print more coins any time, so it has no reason to cheat.
If the bank issues coins in return for real money, and then refuses to accept them back, it's gained the amount of money it just ripped off. Doing this often enough to be noticed loses reputation, of course; you can sometimes get away with it if you're a government central bank and get a law made saying you no longer have to pay back silver for those paper dollar notes. On the other hand, printing extra coins doesn't get you anything, since nobody gave you any real money for them. Of course, if you can start up a big bank in remailer-space, and get lots of depositors, but nobody knows where you are, you can ignore the damage to your reputation by ripping off all your depositors at once and forwarding your email to Argentina, just as bank-embezzlers occasionally abscond with the whole pile.