steps to reproduce:
1. hold any sword/armor
2. run 

/enchant @s minecraft:unbreaking 1

3. run

/enchant @s minecraft:unbreaking 3 

expected result:
the new level should override the previous level

actual result: 
"Diamond Sword does not support that enchantment message"
(this message is also ambiguous! a second message should appear if conflicting enchantments are present, especially with the addition of data driven enchants, which aren't as obvious to find the conflicts as vanilla ones)