I've just come across the same problem again with someone else using 'Mick', which the Nickname allocated to me by PC when the new forum started. I've just come back from a week away and have found that I've been posting entries apparently.
I have e-mailed PC about this before and have been told that the techies would look into it, but I've never received a reply.
This is totally ridiculous on a forum to allow more than one user to have the same Nickname. No-one knows if they're speaking to the same person or not.