Comment 2 for bug 266263

the conflicts occur when a user cross-posts a message to
two lists. the newsrunner won't turn it into a cross-post,
instead it tries to post it separately to both groups, and
the news server rejects the second one for a duplicate
message-id (tested with innd).

of course, you could say this behaviour is a bug too :) i'm
not sure what the best way to group cross-posted messages
is, since they won't be received by the same mailman
instance. (cancelling the first and re-posting to both
groups might work, but i don't know if inn will still
record the message-id and not allow it to be reused.)