The 'Doctor Who' episode "Let's Kill Hitler" provided another splainin for recastaways and doppelgangers - shape-shifting spaceships like the Teselecta.
Here's a description from the TARDIS wiki:
The Teselecta, designated Justice Department Vehicle Number 6018, was an android staffed by a crew who had been miniaturized by a compression field. It could travel through time. It was used to locate people deemed guilty of crimes who had escaped punishment. After finding them near the end of their respective timelines to avoid changing history, the crew would administer their own punishment, usually in the form of some non-lethal, painful torture, so that afterwards they could die in the manner recorded by history.
The Teselecta could resemble anyone or anything whose likeness it has scanned. During its time in Berlin, 1938, it took on the forms of a janitor, a Nazi official, a Nazi soldier (with and without a motorbike), as well as Amy Pond and River Song. A member of the crew stated that it had once taken the form of Rasputin, but with green skin (DW: Let's Kill Hitler). At some time in the future the Teselecta took the form of Father Gideon Vandaleur, a deceased member of the Silence, and at his request on 22 April 2011, the Eleventh Doctor. (DW: The Wedding of River Song) It was even able to mimic regeneration at Lake Silencio. (DW: The Impossible Astronaut) The Teselecta had a lot of damage resistance, as the Doctor declared that he "barely got singed" when his 'body' was being cremated at Lake Silencio. (DW: The Wedding of River Song)
Since that vehicle was designated Number 6018 in the Justice Department fleet, there could be plenty more of the same design. (Not that all 6018 vehicles were shape-shifting spaceships.)
So those other vessels similar to the Teselecta could be out there, at any point in Time, impersonating various TV characters - especially those one-off guest shots played by the same actor but playing different roles in the same series. So long as there is no extensive backstory, we could make the claim that the disguise is the default program.
An example that immediately sprang to mind was that of all those characters played by Vito Scotti of 'Columbo' - among them the funeral director, the bartender, the sommelier, the tailor, the wino..... For some reason, that particular Justice Department vehicle needed to keep tabs on the rumpled detective.
As to why other characters - like Lt. Columbo - couldn't notice that they all looked the same, the standard Toobworld splainin is that there must be some differences that can't be seen by the Trueniverse audience.