I think "If I met Duo, I would be very happy", "If I meet Duo, I would be very happy", "If I meet Duo, I will be very happy", If I meet with Duo, I will be very happy" should all be marked as correct. This 'If' lesson has a lot of very precise answers and doesn't allow many answers. I always spend most of my time just trying to get the wording exactly right in English even though I know the meaning of the phrases...
This English translation is incorrect English as the comments below assert. (English native speakers notoriously use conditionals incorrectly.)
Here is how it works in English:
Real condition/real facts referring to the present: "If you are my daughter, I am your mother." "If this is number 3, Main Street, then this is the right house." "If it is raining, take your umbrella." "If I am dreaming, please do not wake me up."
Real condition/real possibility referring to the future: "If I meet Duo I will be very happy." Further example: "If I get this job, I will be able to buy a car." (NOTE: we never say "If I will meet...." using the future tense, in the "if" clause, but only in the result clause.)
Unreal condition/hypothetical situation in the future: " If I met Duo I would be very happy" OR "If I were to meet Duo I would be very happy." Further example: "If my grandmother had wheels she would be a wagon." (proverb) (NOTE that this refers to present time, although the verb form looks like a past tense; it is in fact the subjunctive.)
Unreal condition referring to the past (it did not happen): "If I had met Duo I would have been happy" (refers to past, with result in the past) OR "If I had met Duo I would be happy" (refers to past, with result in the present). Further examples: "If I had known he was your father, I would have spoken to him more politely." "If he had drunk the poison he would be dead by now."
I think part of the confusion in the comments may arise from the fact that while in English we distinguish "real conditions" and "unreal conditions", Turkish apparently has several options and shades of possibility, which do not "map onto" our English patterns exactly. The book "Turkish: An Essential Grammar" by Aslı Göksel and Celia Kerslake is very helpful in explaining this in chapter 23.