Grindstone can merge a tool even if the tool is not used or the durability is not low

I was trading the tool smith villager to get the final offer using emeralds, then my inventory is full of iron axe, i was going to get rid all of my iron axe that i buyed, then i thinked that i can use the grinder to get rid or merge them into one and i was shocked that you can megre the same tool using the grinder even the durability is not that bad or the tool is not even used. please fix this bug, thank you