If one is davening SE and has a safek whether he already said the chasima (conclusion) of a certain bracha, or did not say it, what should he do? On the one hand, one cannot make a blessing in vain (by repeating what he might have already said); on the other hand, if he skips it, his whole Amidah will be invalid.
Usually this should not happen, but if one is davening fast and not really paying attention, it might happen…
One who is not sure if he finished off a bracha has the same rule as one who lost his place in Shmone Esrei, as they are both unsure if they said certain parts of Shmone Esrei. He would have to say the bracha he is doubtful of saying, before continuing his Shmone Esrei.
Kehilas Yaakov, Brachos 13, Orchos Rabeinu Vol. 3 pg. 207 in the name of the Chazon Ish, see Pischei Tshuva to O:C 119. cf. CHaye Adam 24:21. While one generally can’t say a bracha he is unsure of as you mention, here he must do so as otherwise every subsequent bracha will be doubtful.