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 sefirat Ha'Omer , 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 lekula.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?
To clarify the question: The Shulchan Aruch (489:7) clearly says not to make a blessing in the day if you forgot to count at night (the case of doubt, since some opinions hold you have to count at night). The case that needs to be explained is why you can make a blessing on the following days (489:8) according to those opinions that you never make a blessing in case you are in doubt whether you are fulfilling a commandment.
The easiest answer is that the poskim who hold that you don't repeat the blessing in case of doubt would hold the same with regard to counting the omer (against the Shulchan Aruch). This is what the Gra (489:8) writes:
וכ"ז לדעת הפוסקים דבספק דאורייתא מברך
However, the original rationale of the Shulchan Aruch might suggest that counting the omer is an exceptional case. The opinions that hold that the entire counting is one commandment base it on the wording of שבע שבתות תמימות, seven complete weeks. If you miss counting one night but still do it in the day, your counting can still be called "complete" because you did do it during the day, even according to the view that you didn't fulfill that night's obligation. This is apparently the view of the Terumat Haddeshen (quoted in Bet Yosef 489:8), who is the source of the rule brought in the Shulchan Aruch:
דמיקרי שפיר תמימות הואיל ולא דילג יום אחד לגמרי