I read a book back in the '90s about a school-aged girl who runs into another girl who looks exactly like her and is apparently killing people with psychically induced heart attacks. The main character finds out that she also has psychic powers (she figures this out when she comes under attack and constructs a "wall" in her head to stop it) and is one of multiple girls from an experiment. Any idea what it could be? I've been hunting for any kind of clues about it for ages now. I though it might have been an RL Stine book, but couldn't find any that match the description.
Is it Eve from the X Files novel series? It fits some of the criteria (90s, girl from experiments) but I think the killing was with digitalis rather than through psychic powers.