And, Flash Gordon was a knock-off of John Carter. Initially, they were trying to get the rights to do a Carter comic strip and failing that, converted it to Flash Gordon. You watch the 80s Flash Gordon movie and the more recent John Carter movie and you can see how the stories follow the same beats, with the same climactic scene.
That reminds me that the 80s version of G.I. Joe was originally going to be a toy line based on Nick Fury and S.H.I.E.L.D. but when the license lapsed, Hasbro dug up their old Joe property and repurposed it. S.H.I.E.L.D. became G.I. Joe and Hydra became Cobra.