Given two items:
<italic><rarity:blue>Chipped FlameVenom Fang | Sharpness 1 | Fire Aspect 1 | Unbreaking 5
and combining them in an anvil, the result has
SH2/FA2/UB3 | cost: 30 levels
This downgrades the Unbreaking enchantment.
I propose that when high-level enchants (meaning higher than possible normally in survival) are present on an item, and two identical copies of that item are combined in an Anvil, that:
1. The result is identical to the inputs, durability added
2. The cost is reduced
Or, a second result is available with the above changes.