Can't import sample email addresses into separate groups

I have 3 email groups. 2 of them have separate email addresses in them. The 3rd is for people that have registered for an event so this group requires adding email addresses that are in the first 2 groups. When trying to import the 3rd list, the system doesn't import email addresses into that group that are already in the other 2 lists. I understand that you can assign email addresses to different groups manually, but that would be extremely time consuming in this case. I would like to be able to import the 3rd list and have the system up the database to include that group for an existing email record. Please let me know how this might be possible. Thanks.