Send has a function where duplicate recipients can be removed upon sending. This is known as deduping.
To use this function you will first need to select two groups that share a number of contacts.
Deselect one of the groups, this will then take out all of the duplicate contacts from the first group.
When a Campaign is sent out immediately, the figure specified next to Current recipients shows how many contacts will be receiving the Campaign.
Please note that when you schedule a Campaign and have selected both groups, the figure will return to the total of both groups before it was deduped.