לֹ֥א תִשָּׂ֛א אֶת־שֵֽׁם־יְהֹוָ֥ה אֱלֹהֶ֖יךָ לַשָּׁ֑וְא – You shall not take the name of the Lord, your God, in vain.
There are two rules – Safeik D’oraisa L’chumrah and Safeik Drabbonon L’kulah. If you have a doubt if you shook the lulav and esrog on the first day of sukkos, you would have to do it again as it is a mitzvah D’oraisa; however, if your doubt is if you made the brocha before shaking the lulav (but you do know you shook it) you would not make another brocha as that is a safeik in a drabbonon.