I'm not sure you can call Dual a horror movie, it's more sci-fi with some thrills, and comedy, and a little horror mixed in. (Once again, that horror involves the accidental killing of a dog. Why does this countdown include so many dog deaths???)
If you've ever wondered what Nebula may be like should she exist in the real world, Karen Gillan in Dual may provide that answer, as she plays both Nebula and Dual's Sarah much the same way: deadpan, emotionless, and a little confused by it all.
Set in what may be the near future, or just...Canada, the basic plot centers on Sarah, who has just found out she has a Terminal Illness (not defined beyond that) and if she wants, she can create a clone of herself to help her loved ones deal with the loss. She agrees to it but...things don't turn out as planned.
The plot is really similar to the 2021 Apple TV+ movie Swan Song, though that one was a straight up drama, and this one veers into the realm of comedy more than once. They'd both make a great double bill, and you can decide which one is the clone of the other.
I watched Dual on Hulu.