Rulings

If a spell has multiple instances of affinity, each one applies. For example, if you somehow control two Pearl-Ear, Imperial Advisors and you control two Auras, each enchantment spell you cast will cost {4} less to cast.
A creature you control that's the target of an Aura spell isn't modified unless it already has a counter on it, an Equipment attached to it, or an Aura you control attached to it.
A creature that is equipped is considered modified no matter who controls the Equipment that's attached to it.
A creature with a counter on it is considered modified no matter what kind of counter it is or which player put it on that creature.
An Aura controlled by another player does not cause a creature you control to be modified.
If you put an Aura on an opponent's permanent, you still control the Aura, and it still counts for your spells that have affinity for Auras.