I have this problem with the Apple Mail application. I have configured it so that it will download mail from two different POP accounts.
I've noticed lately since the 10.3 upgrade that it'll download certain messages twice! Doesn't seem to be a consistent pattern.
I do notice that for notifications from Livejournal, it'll almost always download the message twice, so that I'll have two copies in my inbox. I'd have to check, but I think it'll sometimes do that from both POP accounts.
This gets kind of annoying b/c I don't want to have 20 messages, 10 of which are repeats, accumulating every single day. That's probably not a whole lot of HD space, but it's a bug I want to squash nonetheless.
Anybody have this same issue and resolved it?
I like Apple Mail...I just wish it was robust like Eudora. Certainly is one of the better looking mail readers out there.