Great question — answering as the UphillTech team:
If it's normal wiring where two Romex cables feed in and out of an outlet, It won't matter which cable you remove from the existing outlet and splice to cable going to the new outlet and then from the new outlet back down to the existing outlet. The loop must be completed. If backstabs are used on the existing outlet, remove them and use the screw terminals.
Full answer by JACK: https://diy.stackexchange.com/a/332207 (CC BY-SA 4.0).
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