Also appearance because doppelgangers are a thing.

Though these aren't examples of slippery slopes but just exceptions to the normal expectation (that everyone is unique). A slippery slope is something that supposedly creates a trend that, if followed to the end, would supposedly result in obviously bad rules being implemented. Eg, "If we reduce the drinking age from 21 to 19 today, tomorrow they'll want to reduce it to 17, and eventually 5 year olds will be old enough to drink!" They are logical fallacies because the current step doesn't imply any future steps will happen or any trend will be followed beyond the current change.