This is a remake of the live-action Season 1 episode #6 "SKIN".
During the search for their father, Sam and Dean investigate a small town where a series of strange murders have occurred. They soon discover the murders have been committed by a Shapeshifter, a monster who sheds his skin and can appear as anyone. Sam and Dean separately go to search the town but when Dean meets back up with Sam he becomes suspicious of his "brother's" true identity too -- Why isn't Sam mad that he called him "Sammy"? Turns out the Shapeshifter has taken over Sam!!