Forgive me, I can’t remember the episode names so I’ll just leave a brief summary:
Example 1.) Voyager: Seven and the Doctor are the only ones left awake as the rest of the crew sleeps as the ship passes through a nebula. Seven starts to lose her shit from the isolation. Enterprise: T’pol and Dr. Phlox are the only ones left awake as the ship passes through a nebula (or whatever). This time it’s the doctor that loses his shit. At the end, we learn that T’pol was never really awake, but rather an illusion.
Example 2.) NG: Riker gets accused of murder after fooling around with some researchers frisky wife.
Voyager: Paris gets accused of murder after fooling around with some researcher’s frisky wife.
Pretty sure there’s more, just can’t think of them off the top of my head. Anyone else?
I can’t recall any examples, but it seemed to me throughout the first season of Star Trek: The Next Generation they were redoing ideas from the original series. It got so bad that I stopped watching altogether for years.
I presume we’re not counting the blatant ones like “The Naked Truth” / “The Naked Now”? I guess that by that time in the show’s run, the writers had just run out of ideas.
How about this: A freak accident splits (Kirk)/(Torres) into (his)/(her) (good half and evil half)/(human half and Klingon half). (He)/(She) struggles to keep (his)/(her) other half out of trouble, but in the end realizes that (he)/(she) needs both parts of (his)/(her) personality, and is recombined back into a whole person.
The Changeling vs TMP - old Earth space probe encounters an alien probe in deep space, gets combined with it…the combined entity returns to Federation space, ‘cleansing’ organic ‘infestations’ and ultimately threatening Earth, until brought to heel by a member of the Enterprise crew. It ended better for V’Ger than for Nomad, but still…
“The Naked Time” was a major step in the TOS; it defined Spock as a character and was the beginning of his popularity. “The Naked Now” was deliberately trying to do the same thing for the new crew.
I can’t remember the name, but TOS episode where the "alternative’ Universe with the much more ‘barbaric’ Kirk and crew was re-visited (twice, IIRC, or in two episodes) during Deep Space Nine.
There were 5 Mirror Universe episodes in DS9 - One each in Seasons 2, 3, 4, 6, and 7.
It was also revisited in the Enterprise two-parter In a Mirror, Darkly.
But all of these are sequels (or a prequel for IaMD), not repeating the same plot. (Crossover did involve an accidental crossover, where Cisco interfered with the MU’s development, but that’s an extremely vague similarity.)
Q showing up in every franchise. He had a few good episodes in TNG, then he popped up on DS9. Although I appreciated it when Sisko punched him in the face. Maybe if Riker or Worf had done that, he would have left the Enterprise alone. Then he showed up on Voyager. The first time, with the other Q who wanted to commit suicide, was decent, but the episode with the Q civil war was beyond stupid. Then he comes asking Janeway for help in disciplining his son.