Killing a person, then pretending to be them. This can crop up in a number of circumstances. A common ploy in murder mysteries is to have the killer impersonate the victim in order to throw off the time of death. The very first episode of Columbo
used this plot. Can be used to create dramatic irony if the audience is made aware of the ploy, or a plot twist if they aren't. Obviously becomes a lot easier when the killer is a 
- Monk has had this happen at least once.
- Ditto Jonathan Creek.
- Sylar from Heroes did this after he killed Zane Taylor. And a couple of other times.
- Happens in the Pushing Daisies episode Pigeon, though the death was an accident there.
- Quite a few Ace Attorney murderers have used this ploy.
- A key plot twist in the John Dickson Carr novel The Hollow Man/ The Three Coffins relies on this.