NBC had a problem with doppelgangers in Star Trek: The Original Series' first year scripts

Star Trek: The Original Series was trying to find its space legs, so to speak, in season one, and there were scripts that seemed far too similar to suit NBC, the network that aired the series. The main issue the network had with a handful of The Original Series' first episodes was the use of duplication.

In "The Man Trap," we have the salt vampire who can (and does) become anyone, including the woman who owned Dr. Leonard "Bones" McCoy's heart at one time. But the creature also transforms into Dr. McCoy himself. Then along comes "The Enemy Within," which gives us two versions of Captain Kirk, the good side and the overly-aggressive bad side. "What Are Little Girls Made Of?" gives us two Kirks again, although one is an android. And, finally, "The Alternative Factor" includes two different versions of Lazarus.

All of these doppelgangers proved to be a bit much for NBC, and, according to WhatCulture, the network let the producers of The Original Series know that no more scripts involving duplicates would be approved.

The use of duplication gave us two strong episodes, "The Enemy Within" and "What Are Little Girls Made Of?" Yes, they both had two Kirks, but the plots were so varied that they couldn't be mistaken for one another. As far as the other two episodes go, they weren't the strongest in the series' first season, but they were unique.

Overall, the doppelgangers weren't so overused as to have been confusing or irritating...at least not to fans. On top of that, this was a new science fiction series so NBC should have expected the weird, even if it meant two Captain Kirks. As far as I'm concerned, "The Enemy Within" is one of the stronger episodes of the first season, and it had a powerful message of the sides we carry in all of us.