In older movies where an actor/actress plays the role of two characters, what filming techniques are used that allow both characters to be visible in a scene?

In on watching Cronenberg's Dead Ringers and I'm curious as to what filming methods are used. In a lot of the shots it's obvious that it's just a lookalike as only one face is visible at a time, but in some shots and scenes both of Jeremy Iron's characters faces are visible.

I'm assuming CGI is used in modern films?

Submitted February 17, 2019 at 07:18AM by MisterCreepz http://bit.ly/2EeprcB