Arena by Fredric Brown

"Arena" by Fredric Brown was adapted (sort of) twice for TV...

...first, in 1964, on the Outer Limits as "Fun and Games", starring Nick Adams.

You'll note a number of differences between the story and the episode, including a pair of new characters, a mate for the alien, and a woman with a hidden past who works with the hero!

A year later the story was reused, on Classic Star Trek as "Arena"...

...except it wasn't...exactly.

Gene Roddenberry's right hand man, producer Gene Coon, had submitted a story about Capt. Kirk fighting an alien starship commander to see whose ship would survive.

When the story was fact-checked, it was discovered that Coon had inadvertantly used numerous elements from Brown's story, which Gene had probably read years earlier!