Not talking about blatant ripoffs, rather about movies that seemingly got their ideas from other movies and only YOU noticed. For example, watch Joseph Reuben's thrilling cult film The Stepfather
and follow that one up with Douglas Hickox' made for cable TV thriller Blackout
Similarities: atmospheric small town w/ autumn leaves setting (Identical backyard scene), domestic slasher theme, assumed Identities theme and the giveaway: exact same foggy bathroom mirror scene.
Verdict: The Stepfather
contains at least 70% Blackout