Other people.

Plot: A kid sees his brother being replaced by a look-alike and act-alike alien. Cue long quest to rescue brother. In the end, decides that the aliens, who are perfectly mimicking everyone they replace, down to memories and thought processes, might as well be the people they are mimicking.

What's the difference between the real thing, and something completely indistinguishable?

(Note: I now realize this also comes up in Ghost In the Shell: 2nd GiG. See episode: "DI: Face – MAKE UP")