The bug

If an item has the same enchantment multiple times in 1.20.4 or earlier, and this item is upgraded to the latest 1.20.5 snapshot, subsequent instances of that enchantment disappear without a trace.

How to reproduce

A world containing a netherite pickaxe with two instances of Efficiency V is attached to this ticket.

1. Open this world in 1.20.4
2. Look inside the left chest
3. Open this world in the latest snapshot
4. Look in the left chest again

Expected results

In the latest version, either the two copies of the enchantment would remain, or they would be combined additively into one instance of a stronger enchantment (e.g. Efficiency II + Efficiency II = Efficiency IV).

Actual results

One of the instances of the enchantment is completely wiped from the item.