In siman samach zein in Mishnah birurah there is a disagreement if someone has a safek by say lulav and therefore you must repeat the action whether you repeat the bracha as well. Yet, by sefira if someone has a sefek whether he counted, the Mishnah birurah says he counts during the day and continues counting the rest of the days with a bracha. What is the difference, please provide sources as well
See the M:B 489- 34 & 38 who explains the reason why sefira is different. It is because if he counted by day or if he has a safek if he counted, then it is a safek sfeika, maybe he was yotza, and maybe we pasken that each day is a separate bracha, therefore on a safek sfeika we are lenient that he can then say the bracha.