In Spanish, you can use a form of "ir" + "a" + the infinitive of another verb to form an immediate future tense. (It's equivalent to saying "I'm going to X" in English.)
Voy a ver a la doctora. = I'm going to see the doctor.
Vas a comer las zanahorias. = You're going to eat the carrots.
Va a preparar la cena. = He's going to make dinner.
Duo's set up to automatically accept "I'm" as a direct substitution of "I am," so it shouldn't ever correct you to "I'm" when you type "I am." When something like that happens, the most common reason is that there's a typo somewhere (for instance, somebody might type "I sm" or "Im am"), and Duo doesn't recognize the "I am." In that case, it'll look for the closest possible translation, and if it decides you were trying to type "I'm," that's what it'll correct you to. (The other possibility is that it was a new bug or passing glitch.)
Anyway, if something like that happens, your best bet is to double-check your response, and if there are no typos, use the "report a problem" button and the "my translation should be accepted" button to report it. That way, the system will save your answer, and the Spanish team can figure out what the problem was.
It's called the "personal a." In Spanish, if the direct object of a sentence is a person (or a person-like animal, like a pet), you always use "a" before it. (If the direct object is not a person, you don't.) So: voy a ver a Juan, voy a ver a mi hermana, voy a ver a la doctora. But: voy a ver la película.