Since 1.8 there is a new enchanting system (as you probably know). This made renaming an item cost 1 level, but when you repair the tool you renamed, it still doubles the cost, while you didn't repair anything. Because renaming your item makes it so you can not repair your tool as many times anymore, makes it not worth anymore to rename. While renaming is just for the fun and only 1 level.
A fix for this bug could be to remove renaming from the repair and just set it to 1 level cost, as it should be.