Text file merger?

[Edo]

I have several files with lists of words, one per line, in alphabetical order.
I need a program that will mix all the files (about 4), then check if any line appears twice and delete any line (word) that appears twice.
It should then go on to put what is left again in alphabetical order again, so it is left with a list of words (none of which appears twice) in alphabetical order, one per line.