If one is unsure as to whether he recited a blessing on the food, then it may not be repeated.[2] [However, if one has a Shehakol food available then he should say Shehakol over it and have in mind to exempt the food with this blessing.[3]]

• Example: Suppose Yosef is about to eat an apple and cannot remember if he already said the “Borei Pri Ha’etz” blessing. Rather than repeating the blessing and risking saying it in vain, Yosef notices a piece of candy nearby, which requires the “Shehakol” blessing. Yosef says “Baruch Atah… Shehakol Nihiyah Bidvaro” over the candy, eats a bit of it, and has in mind for the blessing to also cover the apple. Yosef can now eat the apple without worry.

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[1] Admur 167:12 [regarding Hamotzi]; Michaber 167:9; 209:3; Ketzos Hashulchan 37:10; 55:10; Encyclopedia Talmudit Erech Brachos Vol. 4 p. 303; Piskeiy Teshuvos 209:6; See Chapter 3 Halacha 5??! *Source sheets

If one is almost certain that he did not say it: See Tzemach Tzedek O.C. 1:3-7 that according to Peri Chadash whenever the Safek is more tilted towards that he did not say the blessing then it may be said, however one may only rely on this regarding the blessings of Shemoneh Esrei

[2] The reason: As the general rule by Rabbinical commands is Safek Derabanun Lekula, and hence blessings which are Rabbinical would not be required to be repeated in a case of doubt. Furthermore, one is not even allowed to be stringent upon himself and say the blessing anyways, as blessings may only be recited when mandated by the sages, and hence if he were to say the blessing it would be a questionable blessing in vain. [Admur 167:12]

[3] See Ketzos Hashulchan 37 footnote 24