[Exim] Unique ID's not unique

I am having messages generated to me through one particular smtp which have
the same unique id as shown through the uidl command issued by my MUA. I'm a
novice end user, but have some understanding of how Exim works. My question
is what could cause Exim to begin to generate identical 25 character id's on
upgrade to v 3.3#1?

The result is that if I leave my messages on the server, my MUA will not
download after the first set from this sender, as the pop3 database on the
client computer shows the messages have all been downloaded already.

Thanks for any help you can offer.