Duplicate Pickaxe in grindstone

I had 2 unenchanted pickaxes, 1 had almost no durability left and the other had more then half over, and I put them in a grindstone. I tried to repair it and it worked, but the two pickaxes stayed in the repair slots. So I tried to fix them again and it worked again, like this I was able to create 3 new pickaxes out of nothing. I hope it gets fixed, thank you.