Books can be combined in an anvil even if the resulting enchantments are the same as the original one

The bug

When trying to combine any already enchanted item with an enchanted book with a lower enchant level, the combination will work even though the item doesn't actually change (apart from future higher combining costs).

To reproduce

1. Get iron leggins and enchant them with Protection III
2. Get an Enchanted Book with Protection I
3. Switch to Survival mode
4. Try to combine both in an anvil
   → That is possible, even though the original and the resulting items are the same