Since I'm using the program in swedish maybe it is called something else so bare with me.
It is a relativly new feature so maybe you need to upgrade to latest version of FTB.
Under Tools in menu bar where you also can check for updates you find it: Check for Douplicates or something like that. It finds possible douplicates and you can edit left and/or right person, not merge them. You can save it to a file.
