It'll be in an upcoming piece we're working on about the fine line between rip-off and homage. But as
@Harvey posted here years ago:
Actually, de Forest Research caught the similarities. This is what they said in their August 11, 1967 research report (note that this report indicates that Desilu was notified of the similarities earlier, when the story was purchased):
It’s a green-gold ball of fluff, a tribble. – The premise of a small, featureless, fluffy, purring animal, friendly and loving, that reproduces rapidly when fed, and nearly engulfs a space ship, is the major plot complication of the last one third of Robert A. Heinlein’s novel, The Rolling Stones, published by Scribner’s in 1952. Use of this plot line which parallels the story line of the novel could lead to legal complications. Mr. Heinlein might claim that his property was rendered unsaleable to television and motion pictures because the story line had appeared on Star Trek. The similarities between these two stories were pointed out when the story outline was first presented. It might be advisable to contact Mr. Heinlein and negotiate purchase of property.
Heinlein's authorized bio states that he'd just been through a plagiarism lawsuit and he caved on "Tribbles" only because he figured it was a one-time thing and would be forgotten. He considered the ongoing merchandising of them to a betrayal of the gentleman's agreement he made with Coon.
(LINK)
And I looked through his papers and saw the script they sent him.