First. This topic is a bit complex. Here are some relevant parameters:
- Host or guest for meal? A person who is invited does not have to bless twice even if the second fruits were not at his disposal (summarized from Bet Yosef OC 179, paragraph 5).
- Was the food already inside the house or not? If this food is a gift that comes unexpectedly from another house, even guests need to bless (summarized from Bet Yossef 177 paragraph 5 in name of Sefer Hapardes left bottom)
- Was the food previously expected or not (Was it a part of the menu)? If it was planned, despite it comes from an other place, no additional bracha is required because it is included in the scope of the first beracha as the suggia of 'Amat Hamayim' (water from a river, when you are blessing, the water you would drink is still not next you (Gemara)).
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If I blessed on some of the items of food that were in front of me, and suddenly realized that this bracha also applies to another foods in front of me, do I need to make a new bracha?
No. Not only if you are a guest, but also if you are the owner, because it is in front of you at the time of the blessing.
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If not required, would it be permissible?
No. The one who says a blessing , that is not necessary transgresses "Lo Tisa" (pronouncing the name of G-d without need) from SA OC 215, 4 (Gemara Brachot 33a)
ואמר רב ואיתימא ר''ל ואמרי לה ר' יוחנן ור''ל דאמרי תרוייהו כל המברך ברכה שאינה צריכה עובר משום {שמות כ-ז} לא תשא
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But then, before I said HaAdama, I looked up the bracha for mango and found it was HaEitz. Should I say another HaEitz on the mango?
To simplify, let us assume that you know that Mango is Ets, but you intentionally don't want to include the mango in the Bracha, I think that you can make a second Bracha for the mango.
See Berachot 41.1
however, when their blessings are not identical, everyone agrees that one recites the appropriate blessing over this one and then he again recites the appropriate blessing over that one.... They disagree [for ower question the disagreement between Rabbi Yehuda and Chachamim is not relevant] with regard to which blessing precedes the other).
Your question includes an other parameter. You though Mango is Adama. But if you knew mango is Ets, you would have to include it in the first blessing... more parameters need to be be discussed.
An additional side of the example of fruit salad is mentioned by @DanF, concerning majority, Ykar and Tafel (main meal and condiment). it is a separated topic that should to be treated, but I will not talk about it. Let us assume that you have in front of you a mulberry tree fruit and a mango, not mixed.
- For the retroactive effect of a bracha, stricto-sensu, the only case I remember in Rosh Brachot 6, 27 in name of Baal Halachot Gedolot is that Birkat Hamazon is a valid blessing Acharona for the wine of the Zimun, that will be drunk after the Birkat Hamazon. But Rosh and poskim in Bet Yosef OC 190, 2 do not agree for Halacha. See also SA OC 172.1 in the Rama. If someone has a drink in his mouth and remembers that he didn't bless Beracha Rishona, he needs to swallow it and afterwards to say Beracha Rishona.
5.If you want to eat a single fruit only and blessed for it, but this fruit falls and became inedible, so, you take immediately a new fruit, even entirely similar. You need to bless again because it was not in your intention to include it when you blessed. See SA OC 206, 15