Duplicide.
Not really, I just made it on the spot from the usual suffix and the Latin word root for double.
Alterantively: Sosicide - uglier, and does not come (directly) from Latin but from the Italian, French, and Spanish word for double: sosia, sosie, sosias.
Tonight it’s linguistics night.
EtA suggested reading: Jose Saramago’s “The Double” (" O Homem Duplicado")
