A person you’re having sex with with who looks unmistakably like someone else that you’ve already had sex with. The principle of the doppelbanger requires you to finish as quickly as possible given the inherent levels of danger with mating both sides of the doppel coin.
Hey I absolutely threw one into a doppelbanger last night, total double of Julie from last summer. Needless to say I gave her the beans and then left quickly before she killed me.
Someone who has sex the same way as a previous sexual partner.
"Dude I hooked up with Jess last night and it was like I was banging my ex. They were like doppelbangers or something."
A look-a-like of somebody that you used to date (or have sex with). Similar to the original meaning of "doppelganger" - known as a strangely similar counterpart of someone.
"Dude! Is that your ex girlfriend?" "Nah, just a doppelbanger of her"