Cloning for Colonization?
In the three seasons of TOS, we see many extras and guest stars reappear in different roles, but, one obvious in-universe explanation is cloning. Maybe humanity could not meet the high demand for people to support the massive human colonization suggested in the series. We see many colonies and even whole planets populated with billions of humans. I think it unlikely that this could occur only over ~200 years of warp flight. Assuming natural births could not fully meet the demand for people, human culture started making test tube babies. If you are going to make a human for colonization, you may as well select human genes from quality individuals that give preferred characteristics. Cloning became perfectly acceptable with no prejudicial treatment of these individuals evident in the show. Several clones have attained high status such as:
What are your thoughts on the possibility that cloning is common in TOS?
In the three seasons of TOS, we see many extras and guest stars reappear in different roles, but, one obvious in-universe explanation is cloning. Maybe humanity could not meet the high demand for people to support the massive human colonization suggested in the series. We see many colonies and even whole planets populated with billions of humans. I think it unlikely that this could occur only over ~200 years of warp flight. Assuming natural births could not fully meet the demand for people, human culture started making test tube babies. If you are going to make a human for colonization, you may as well select human genes from quality individuals that give preferred characteristics. Cloning became perfectly acceptable with no prejudicial treatment of these individuals evident in the show. Several clones have attained high status such as:
- Admiral Fitzgerald and Commodore Barstow
- Commodore Robert Wesley and Cmd. Giotto
- Capt. Ronald Tracey and Dr. Simon Van Gelder
- Lt. Cmd. Ann Mulhall and Dr. Miranda Jones (cloned and genetically altered for telepathy but also resulted in blindness side effect)
- Number One and Nurse Christine Chapel
even
- Capt. James T. Kirk and George Samuel “Sam” Kirk
What are your thoughts on the possibility that cloning is common in TOS?
and raise families. Only now during the TOS era are we seeing women returning to the work place and serving on starships. A new women's lib movement is coming...

; I wanted to keep Spock's
. Alas, foiled again...
)
But I'd have to say it must be a mass of coincidences! Look how many people do look alike today and yet they are no relation in this day and age or in any other and you'll see a constant stream of correlations! It would be a good way to explain it though I guess although Kirk doesn't seem to say anything to Simon Van Gelder about his similarity to Captain Ronald Tracey, or Miranda Jones about her looking like Dr.Ann Mulhall or Giotto as being a Bob Wesley lookalike!!! And he certainly doesn't quiz Admirals Komack or Westervliet about their ancestry or even some of his crew that have duplicates like Lt.Johnson who resembled the sadly deceased Lt.Galloway or the Mr.Leslie brothers who seem to take over when their brother is killed by the Vampiric cloud in Obsession yet he can't get Angela Martine's name right on the Shore leave planet and on the bridge where he calls her Lt.Lisa but then that wasn't really Kirk was it and or Mr.Kyle's name in Immunity Syndrome! 
Thanks for supporting my original post, but now, I'm convinced there is no in-universe explanation. If the plot calls for the same actor to be a different person, then they must be a different person and damn our lying eyes.
-Henoch)