How can I identify films that appear on more than one of my lists

I have lists of films I want to watch that I compiled from different sources, to help me remember why I listed it, for example films flagged up on courses I've done, or recommendations from a particular critic etc. Some films appear on multiple lists, and I'd like to be able to find these, so I can kill two birds with one stone - one film, two lists reduced, you know. How can I cross reference different lists to flag up the ones I'm looking for?