Catspaw is an episode of the second season of Star Trek, the first episode with Chekov that was filmed but broadcast after others as to have it released more in line with Halloween.
Ok so this episode is indeed a classic, you have probably heard the elements of the story before “transporter accident” and “evil duplicate” well this is where those tropes started with Star Trek.
“Let’s go all the way back to when the series started, no not Kirk – before the first series – to the original pilot.”
The Devil in the Dark is an episode from the first season of the Original series of Star Trek
“A classic – a tale of how misconceptions and reactions can lead to horrible atrocities.”
“So I’m back from glorious battle, and it’s a new year so let’s travel all the way back to a series that thought that 20 years ago we would be having a war against a guy named KHAAAAAAAAAAAAAAAAAANNNNNNNN!!! “