Depending on the method used averaging the files would just result in a file with multiple watermarks (think something like a bloom filter). At any rate the wor

Depending on the method used averaging the files would just result in a file with multiple watermarks (think something like a bloom filter). At any rate the worst that could happen is that the watermark doesn't work, which is a shame for the person that added it but doesn't really affect anyone else. Preventing the attacker from adding arbitrary IDs is as easy as cryptographically signing the ID (requires more bits, but the available space is huge).