[exim] Duplicate Messages

I'm having an odd problem and wondering if anyone else is seeing it.

On rare occasions a user will get several duplicate messages. What
appears to be happening is that the sending server completes the
transfer and Exim accepts it, but the sending server thinks the delivery
failed and therefore resends the same message.

It only happens with specific servers. The duplicates always come from
the same place. And it's rare enough that I can't seem to isolate it.
Most of the time everything works.

Anyone else seeing this?