IIRC, you can almost instantly verify that certain bitcoin wasn't spent someplace else before, which would be good enough. Now, the attacker can use the same bi

IIRC, you can almost instantly verify that certain bitcoin wasn't spent someplace else before, which would be good enough. Now, the attacker can use the same bitcoin for several web forms simultaneously, but 1) the scope of this attack is limited by the time window, and 2) if the server discovers a fraudulent transaction in, say, a few minutes, the spammy comment can be automatically deleted then.