lf I ate olives and I have a sefek by the bracha acronah. Sefek number 1 is maybe I ate a kezayis. Even if I didn’t, maybe a berya (whole item) requires a bracha. By sefiah , if I have a sefek , I count during the day and continue counting the rest of the days with a bracha based on sefek sefika. What’s the difference? The common answer that is given is by the olives the sefek itself is the bracha . So by sefek brachos we go lullaby.But by sefirah , the sefek sefika says I am obligated to count again . Once I count again, I must say the bracha as well. In siman samach Zein the Mishnah birurah brings down if you have a sefek by a Torah obligation, so you must repeat the action , yet there is a disagreement if you must repeat the bracha as well. Why by sefira, the sefek sefika obligates me to repeat action and everyone agrees say bracha, but by a Torah obligation say lulav even though I must repeat action because safek deoraisa lechumra there is disagreement if I must repeat bracha as well.What is the difference.
Here is a possible answer. There is a difference between a safek sfaika, like by sefira, and one safek, like if one can’t remember if they shook the lulav or said kriyas shema. When there is a poskim who say that we don’t say another bracha hold that although m’drabonon we go l’chumra and he has to redo the mitzva, but it still remains because of a safek, therefore the aspect of safek brachos l’hakel will still apply. However, those who say that even by one safek the bracha should be made, they hold that since now there is an obligation to do the mitzva, even though safek sfeika we assume that it is a definite chiyuv, (see Shut Rashba 401) therefore he has a definite obligation to shake or count again. However, when there is only one safek, those it is a safek the bracha should still be made. This idea will only apply to a birkas hamitzvos, that if we have a safek sfeika, we look at it as a definite chiyuv, and therefore it requires a bracha. However, by brachos on food, where the safek is if a bracha is needed here in the first place, then even when there is a safek sfeika, we won’t say the bracha.