"Bought, claimed & gifts" page 21 onwards gives copies of page 20.

(you need > 1000 items in your library to reproduce this. "Free to download or claim" items are handy here, and many but not all can be found at https://itch.io/bundles , https://itch.io/sales or https://itch.io/games/newest/on-sale )

Of course, you will probably not see page numbers for "Bought, claimed & gifts", but rather "infinite scroll" (assuming Javascript is enabled). I first noticed something wrong when the infinite scroll indeed seemed to be infinite - it was in danger of overwhelming my browser. Closer inspection showed that a set of items was repeating, and digging in Developer Tools revealed the JSON the script was fetching - the same result every time saying "page":20 at the start, regardless of the page number requested.

If you don't want to fiddle with Developer Tools, you can see the bug easily in a regular HTML page by disabling Javascript in your browser globally, or just for the itch.io domain, and visiting page 20. Then open the "next page" link at the bottom in a new window or tab. It will be page 21 according to its URL, but notice that it is the same as page 20, and so are all subsequent pages :(