There are no refunds on silver purchases if the item has already been unlocked. In regards to buying Shadowkeep while already owning the season pass, the season pass that comes with the base Shadowkeep will be applied to your account to unlock season 9, the next season.
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Ahh, ok, that means, the next season pass is already paid. Excellent, that work's for me :) So, I have just to buy Shadowkeep. Do I have any prove about that?
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It should unlock automatically for you, but yes, it would be a good idea to keep your proof of purchase handy in case it doesn't unlock when season 9 goes live.