I understand how to add them based on this, however, 

we have a number of contacts whos email address are not unique to the account they are under- meaning,  Joe Blow > j.blow@email.com can be listed under company 1 :> CO 1 ltd and as well as company 2 :? CO 2 Inc.

how can i create a campaigns list that will only allow me to have one and not the other company?

for this particular campagin, it doesnt matter which is chosen. I would rather have a method which isnt me looking down 1,000 + contacts to include or exclude from this list.

any good suggestions?