In the scenes where they are together they place the camera in one place, film both scenes with the two different characters and then impose one scene on top of the other because the camera is in the same place everything is the same except the two characters, it's simple
They had a double play her so that Lindsey could touch and look at her and then they'd switch and do the same things. That, or they had the double, Lindsey played one full twin, and then they took her face and CGI'd onto the double's body.
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