Separate (not remove) duplicates

I have a file with four columns

First Name email Last Name Opt Out

I need to locate which emails are duplicated and create two files.

One file would contain only one instance of every email that is not opted out.

The other file would be any duplicate..

Basically we want to send an eblast only once using the first file..

Then we want to mark everyone in the second file opt out so next time we pull the file we can exclude them.

Note.. I need this to be easily repeated by anyone

Let me know of any questions.