There is no "you" here. Go to your purchase and choose the refund option.

I thought it was not possible to buy a game on Steam Store already owned. The message would come up you already own the game .
The purchase for myself option will be disabled..
Where did you buy a duplicate copy?

引用自 Jaunitta 🌸：

I thought it was not possible to buy a game on Steam Store already owned. The message would come up you already own the game .
The purchase for myself option will be disabled..
Where did you buy a duplicate copy?

Yea, you get a message that says you already own the game. I have a feeling the OP bought a third party site greymarket game or key. They would need to contact whoever they bought it from.