I would like to know the name of the effect that can be heard in pretty much every rap song on earth. It's an effect that sounds like a dual voice, like there's 2 people with the same voice rapping at once. You can hear it everywhere, especially when it's used to emphasize rhymes. What is this effect called (and maybe how can it be done in programs like Pro Tools and Audacity unless it's done using some kind of hardware)? I've searched everywhere but I can't find anything.
Any help will be greatly appreciated.