Dunno if this applies to your situation, but repairing items gets more expensive in XP the more times you do it. It also costs more if the item has a custom name. In fact, I've concluded that custom names are actively a bad idea on high level items for that very reason. If you can't repair an item due to excessive XP cost, what you can do is simply convert it to a new item. To do that, all you need to do is put an unenchanted version of the item in the first slot and the almost-broken enchanted item in the second slot. Combining them will give you a brand new enchanted item with full durability and with all the enchantments from the old almost-broken version.
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