How do I get a refund for duplicate skins?
You go here https://help.guildwars2.com/home.
Then you go to the top right and “Submit a request”.
Make sure you have as much information readily available as possible (serial code, email, account name, character on the account).
EDIT: If you don’t want to click that link (I couldn’t blame you), just go to the top right of the forum page → hover your mouse over “Services” → and then “Support”. It will lead to the same page as my link.