I don't know, maybe it was fixed now, but I found a little bug...
I have a login "root" with nick "KVentz" in my forum. I have some news groups to import. Sometimes a man with nick "root" writes in thees groups. And messages in forum appear like I wrote them. With my nick and my avatar. But I didn't wrote them!
I propose comparing news_nick with forum_nick instead of news_nick with forum_login. Look:
Incoming message from NNTP with nick X.
Checking registered user with nickX ("name" field in DB).
If there is one - write a message as from this user.
If not - register user X with login X and name X (if registering users is on) and write a message as from this user.
Forum do this, but it compares name in NNTP message with login in forum. I think you just need to change "login" to "name" in query to fud2_users table.