You can get it with multiple wow accounts under your battle.net. When you get an achievement that awards an item, it'll give the item to every character that's logged in at the time on that battle.net account .
So say you have a 100 on Wow1, and your twink on Wow2. You log into your twink on Wow2 when you get the achievement on your 100 on Wow1, and both the 100 and the twink will get the hat in the mail.
It's been around for a little while, and works for any items that get awarded directly from achievements. People took advantage of it a lot with Challenge Mode rewards in MoP.
Edit: I'm just guessing that this is how they get Captain's Hat btw, haven't done it myself. I was thinking about trying it but wasn't paying attention and got the achievement.