Noun
(idiomatic) A duplicate or counterpart that acts in a contrary, nefarious, or insidious manner.
As a ploy to discredit the reformist politician, the opposing party had hired a lookalike to act as his evil twin and be seen in a variety of compromising situations.
(networking) A rogue wireless access point installed near a legitimate one for purposes of eavesdropping or phishing.
Source: en.wiktionary.orgEvil twin beers on tap from the Mikkeller co-creator. Source: Internet