Challenges: If it is Summer, choose an opponent's attachment in play. Pay that attachment's printed gold cost to take control of that attachment, and attach it to a card of your choice.

I need clarification as to what "Opponent" means - is this refering to attachments owned or controlled by an opponent

i.e. I have previously taken control of someone else's attachment (say Bastard) and attached to another character. Now I want to pay the cost of an opponents attachment (the same copy of Bastard, which I now control) and move it onto another character

I was told that this was not allowed to do this as I already control the attachment

The text says nothnig about me not being allowed to move an attachment which I already control - it just refers to an opponent's attachment in play

thanks

When a card refers to "your" or "opponent's", it means control. A card should specify if it's play restriction or criteria is "own".