buy one get one more for free?

[pefa]

hello,

 

I am looking for possibility to make promotion in one of my shops - I would like to make a promotion where in case of adding one product to the basket the buyer could add only one more (same item).

 

what is the point?

 

I would like to sell more goods from the store, when selling one item, would like to sell the second one I would like to sell as free, but not always.

 

Is it any module? Any extensions ? Its simple rule maybe.

 

thank you