Since you said it already then you dint have to say another bracha Acharona.
Explanation:
The Bet Yosef (Oh 208) quotes the opinion of the Rabbeinu Yonah by desserts they need a bracha before an after that the Rabbanei Tzarfat understood that Birkat Hamazon does not exempt foods that require Me’ein Shalosh (Al Hamichya) and therefore if one ate oatmeal after a meal you must still say Al Hamichya. Rabbeinu Yonah adds that only applies by other things but if he said Birkat Hamazon on wine it exempts since the Gemara has a Hava Amina that we should say Birkat Hamazon on wine (just people don’t make meal out of it. Also, the Gemara in the first perek of Brachot discusses if someone ate dates and thought they were bread and said Birkat Hamazon it is fine since they also satisfy. Since we see the Gemara in the sixth perek only questioned whether to say Birkat Hamazon by wine and to by dates it seems that wine satisfies even more than dates. Sine Birkat Hamazon exempts dates even more so wine.
It seems from Rabbeinu Yonah that one who ate dates or drank wine and said Birkat Hamazon he fulfills his obligation. However, if he ate oatmeal, that which is Al Hamichya, and is Birkat Hamazon it does not exempt.
So rules the Shulchan Aruch (208:17) Birkat Hamazon does to exempt Al Hamichya than if he ate oatmeal, Birkat Hamazon does not exempt it. However, wine, if he said Birkat Hamazon on instead of al Hagefen it exempts. Soto on dates if he said Birkat Hamazon by mistake instead of Al haetz, it exempts.
However, many Poskim argue of the Shulchan Aruch. The Even Haozer writes: These words make no sense that dates satisfy a person more than oatmeal (or other Mezonot)? The Tur in 211 says that the bracha of Mezonot is more specific and id more important than Haetz. If dates were more important than oatmeal why don we say Mezonot on it. Even more we rule that one who make an oath to not have Mazon . The only thing which are forbidden r eh five grains but you are permitted to eat dates. Even more we rule that one may not say Birkat Hamazon unless he ate a Kazayit of grain, even oatmeal, but eating dates he will not be ale to . Therefore, The Tosfot and the Rosh and Rabbeinu Yonah means that oatmeal is not exempted by Birkat Hamazon if he ate bread dn then after the meal, after he stopped eating bread, he ate oatmeal, which is then requires a bracha before and after. IF he said Birkat Hamazon and didn’t have the oatmeal specifically in mind then it does not exempt and you need to make a separate bracha Acharona.
The Pri Chadash agrees as well that of course oatmeal satisfies more than dates. R’ Akiva Eiger added that even the Ran is of the opinion that Birkat Hamazon is on oatmeal as well.
The Be’er Hagolah writes: my understanding is that it is very difficult to say that and of course oatmeal is more satisfying than dates , as we see the chachamim said to say Mezonot, I would say the Rabbeinu Yonah when he says wine, he also means other satisfying things, it just excludes other things.
Therefore because of Safek Brachot lehakel ne who ate something which the bracha was Al Hamichya and said Birkat Hamazon he fulfils his obligation Bdiavad. However, one who does the opposite did not fulfill his obligation of Birkat Hamazon as the Shaar Hatziyun writes on 168:71
