This is probably a real easy question but I've never had to do it
before and for the life of me I can't figure it out.

I have a table that lists actor id's and title id's for a video
database and I just need to list actor id's where there are duplicate
title id's.
e.g.

actor_id title_id
1 1
2 1
3 2

and the result would be

actor_id
1
2

Any help is appreciated.

TIA