Twins or doubles can be considered a cliche but these plot points don't happen too often in PEPLUMS so I won't describe them as such. Here's a few films with twins, usually one good and one evil, or bad guys morphed into the likeness of our Hero.
Rosanna Schiaffino plays estranged twins in THE MINOTAUR; it's a good role for her.
Gordon Scott fights with himself, in one of the best scenes in PEPLUM history, as evil Kobrak takes his shape, from GOLIATH & THE VAMPIRES.
Richard Greene times two in THE BANDITS OF CORSICA. Good film.
Robert Malcolm plays a dual role in SINBAD AND THE CALIPH OF BAGHDAD