Author’s note from William Pene du Bois, The Twenty-One Balloons (1947):
Just before publication of The Twenty-One Balloons, my publishers noted a strong resemblance between my book and a story by F. Scott Fitzgerald entitled “The Diamond as Big as the Ritz,” published by Charles Scribner’s Sons. I read this story immediately and discovered to my horror that it was not only quite similar as to general plot, but was also altogether a collection of very similar ideas. This was the first I had heard of the F. Scott Fitzgerald Story and I can only explain this embarrassing and, to me, maddening coincidence by a firm belief that the problem of making good use of the discovery of a fabulous amount of diamonds suggests but one obvious solution, which is secrecy. The fact that F. Scott Fitzgerald and I apparently would spend our billions in like ways right down to being dumped from bed into a bathtub is altogether, quite frankly, beyond my explanation.
Wikipedia notes that, as with many pairs of works with surprising similarities, there are also significant differences in tone, intended audience, and other events.