Enchanted books with curses can be stacked using a grindstone

The bug

You can stack two enchanted books together if they have the same curse enchantment on them, by putting them into the two slots of a grindstone.

To reproduce

1. Put down a grindstone
2. Get two enchanted books with the same curse enchantment
3. Put those two books into the left slots of the grindstone
   → You'll be able to pick up a stack of two enchanted books from the right slot of the grindstone

Original description

i was disenchanting using a grindstone, and when i put 2 curse of vanishing books together, i expected it to just put an x and i wouldnt be able to do it. What actually happened was that the two books stacked. You can reproduce this bug by putting two books of vanishing in a grindstone together, then shift-clicking the outcome.