When ther are multiple heroes with Curse of Avernus on the map (e.g. Abaddon and Morphling morphed into Abaddon), the attack speed bonus on cursed targets will always use the highest present level on the map.

For example, if Morphling is morphed and copied a level 1 Curse of Avernus, but then Abaddon levels his curse up to level 2, Morphling's level 1 curse will now apply the level 2 attack speed buff.

The initial weak slow, and the curse debuffs work fine and properly use your spell's level. Only the attack speed buff uses the highest level from any hero on the map.