The most frustrating message you can possibly get when you're working on your Enchanted Gear of Godliness. So your sword has a few (dozen?) enchantments. So you've repaired it a few times. Is that any reason to throw the thing away? NO, I SAY!
In vanilla, whenever you work on an item in an anvil Minecraft updates a hidden NBT tag on the item called "RepairCost". This number is added to the XP level cost of the operation, increasing each time you repair or enchant, until it's too high for the anvil. This mod simply removes that hidden "RepairCost" tag and sets it to zero. So the XP cost is always what it would cost if this was the first time you repaired it.
Note: If at any point you want the XP cost to repair an item to go through the roof again, simply remove this mod and the RepairCost tag will start counting up again.
This mod only works on 1.11. For Minecraft 1.12: download my Tweaks mod and set enabled=true in the "norepaircost" section. All further updates will go there.