Trust me, and trust the interwebs: it's not an American thing, or a Canadian thing, y'all are just misusing the phrase by cramming an 'in' in there that doesn't belong. There's no context in which 'a dime in a dozen' makes sense as 'a rare thing among common things'... a dime among a dozen coins wouldn't be hard to find, there's a 10 in every 12 with 2 remaining... it just doesn't work. I'm not trying to argue or one-up... I'm just a bit of a language buff. If you and your fam are OK with using it incorrectly that's kewl.
