Singling out duplicate data from 2 lists

[Frost]

Ok, so I have multiple lists in text format that I would like to compare to each other and list the items that are duplicates. I've done this unix before but would like to see if there is a site or program that will allow me to do this easier. I'd prefer it just list the duplicate items instead of deleting one of them automatically.

Any input on this is greatly appreciated.