Gas leaked gas from under right rear side

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My 2005 bmw 325i is leaking gas from under right rear side... I already took it to 3 different shops and they dont seem to find the problem, the last place l took it switched a little black canister right under where the leak is, l cant remember the name of it.. Anyway, after that repair it started leaking again, this is very frustrating since l already paid so much money for it and its still leaking gas, please help,,, Note: l notice it usually leaks after filling gas,