Yeah, in fantasy you'd be looking for doppelgangers, maybe shapeshifters or mimics or shades, not clones. I can't think offhand of a fantasy book where that was the core premise, though they do appear as subplots; IIRC, there was one in Elizabeth Haydon's Rhapsody, and the tail end of E. Rose Sabine's A School for Sorcery has mirror-created "clones."
Joan D. Vinge's The Snow Queen has the feel of a fantasy, but is SF at its heart, and features an evil queen attempting to clone herself to extend her rule.
ETA - May have one! Jane Yolen's Sister Light, Sister Dark features a magic that allows users to call a "shadow self", sort of a double/reflection... not sure if that counts?