When a store has a sale - B1G1FREE and you have a coupon B1G1FREE the resulting price will be B1G2FREE. However, there are other scenarios:
Scenario: B1G1F Sale
Buy 4 = 2 FREE
Use (1) B1G1F coupon = 1 FREE
Total = B1G3FREEOr:
Scenario: B1G1F Sale
Buy 4 = 2 FREE
Use (2) B1G1F coupon = 2 FREE
Total = 4 FREEHowever, stores likely will not give you all 4 free, as the ad/cpn state you must "BUY ONE". They could also decide to not give you the 3rd one free as the coupon regulations should be followed.
I've thought about this one over the years and technically, if you follow the coupon directions it would be B1G2FREE, if the store wants to give you the 3rd FREE - Happy day!

(Just remember - they don't have to)