You can have more than 1 enchanted book in one slot of your inventory.
How to reproduce:
When using a grindstone enchanted books or items with a curse retain the curse after using the grindstone, however if you grindstone 2 enchanted books with curse of binding or curse of vanishing they will stack into a stack of 2. Then you can take the stack and combine it with another stack of 2 and keep doing this, you can place it in a slot in your inventory. You can stack them up to 64. If you have more than 64 in your hand you can only place 64 in your inventory slot, and you keep the rest on your mouse.
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We're tracking this issue in MC-140397, so this ticket is being resolved and linked as a duplicate.
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