1. If someone has a doubt about any of the blessings — whether a first-blessing or an after-blessing — he does not say the blessing again, since the obligation to recite blessings is a Rabbinic law and the rule is “A doubt concerning a Rabbinic law is treated leniently”]. If, however, the doubt pertains to Birkas Hamazon (Grace after Meals), and the person ate enough that he was obligated by Scriptural law to recite it, then the person is required to recite Birkas Hamazonagain because of the doubt (Shulchan Aruch and Mishnah Berurah 209).
2. Some say that the Three-Faceted Blessing is Scripturally ordained if the person ate his full. Therefore, if a person ate his full and has a doubt whether or not he recited the Three-Faceted after-blessing, he should eat another olive-size amount of the food of that species and then recite the after-blessing on all of it. (ibid.)
