Did you originally purchase the dragonslayer braum, or was it gifted to you? If you purchased it yourself the first time, then check your purchase history and send a screen shot of that, + a screen shot of the mystery gift notification that displays the skin you received. Those two things combined should get your friend reimbursed.
If it was a gift, then like Eambo said, they might have been involved in the RAF issues and used unfairly earned rp to originally gift you the skin. If that's the case then you didn't get the skin twice, since it would have been removed once the RAF scam was dealt with. If it was a gift, I strongly agree with sending support a message and seeing what they can dig up on the issue.