That's a good point that I hadn't considered. Thanks! I'll keep in mind that I need to be more specific in the message.

Thinking about it, a spurious copy of my message can't be used with future competitions because the PGP signature includes a timestamp (and it is publicly implausible that I would have my system time set incorrectly to the past). But it can be used maliciously with competitions that already exist at the time the signature was created.