Playing on Limitless5 server when using copper anvil to add magic books to osmium leggings I can keep pulling finished items from the finished side of the anvil GUI. When I'm done I get the original item back and the book as well. It will let me repeat this until I run out of enchanting levels. I repeated this dupe with an osmium sword. I haven't tried it on anything else.

Bumping this, since it's still an issue. :)

Waiting for Apotheosis to fix

should be fixed on their side now