This improvement may be obscure to some users, but for others it may be important so I just want to note it here briefly. In the past, CE has de-duped email lists automatically. For instance, if you pull in a new segmented list to an existing list, CE will not allow duplicate email records to be added to the list.
However, if you sent an email to multiple lists and the same address was on two or more of those lists, CE would send to that address multiple times. That is what we have changed.
CE didn't always work this way, and to be honest, I can't quite remember why we made the change in the first place. But, I think it was one of those situations where something that seemed like a good idea at the time didn't end up being an optimal solution for the majority of clients. So, we have changed this back to the original functionality in which, when you send an email to multiple lists at the same time, if an address is on more than one of those lists, it will only receive a single email. Yay!
So, now there are two ways that CE's email module de-dupes automatically for users.
Hope that helps!
Nick