Eutropia’s ability can target a creature that already has flying. It still gets a +1/+1 counter. Multiple instances of flying on the same creature are redundant.

If an enchantment creature enters the battlefield under your control, Eutropia’s ability can target that creature.

Eutropia can be the target of its own triggered ability.

A constellation ability triggers whenever an enchantment enters the battlefield under your control for any reason. Enchantments with other card types, such as enchantment creatures, will also cause constellation abilities to trigger.

An Aura spell that has an illegal target when it tries to resolve doesn’t resolve and is instead put into its owner’s graveyard. It doesn’t enter the battlefield, so constellation abilities don’t trigger.