I assume you mean "which one is correct?". I would definitely say "Buy one, get one free", whereas I've never heard of "Buy one free one". The latter would sort of suggest that the thing is free but you'd still have to buy it, which doesn't really make sense.
Hope this helped
"Your jokes are scarier than your earrings." -Twit
"14. Pretend like you would want him even if he wasn't a prince. (Yeah, right.)" -How to Make a Guy Like You - Disney Princess Style
Hey, sorry if this is a bit late off course the debate between you and your dad But 'Buy one, get one FREE' is best. Although, if you're going to use it publicly, the pretty popular phrase is 'Buy one, take one!', something like that. It's a bit more exciting with that!
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