Learn Swedish: Min mamma och min pappa är här. - My mother and my father are here.

In Swedish, possessive pronouns must match the specific noun they are attached to, not the total number of people in the sentence. Because mamma and pappa are both singular words, they each get the singular min. You would only use the plural form mina with a plural noun, like mina föräldrar (my parents).

I've seen mitt used for "my" in other sentences. Why isn't it used here?

The word you choose for "my" depends on the gender of the noun. Mitt is used with ett words, such as mitt äpple (my apple). Since both mamma and pappa are en words (en mamma, en pappa), they must take the en form of "my", which is min.

Do I have to repeat min before both words, or could I just say min mamma och pappa?

It is grammatically correct and very common to repeat the possessive pronoun before each noun in Swedish, so min mamma och min pappa sounds very natural. However, dropping the second one and saying min mamma och pappa is also perfectly understood, just like in English.

Since "mother and father" makes two people, why do we use är instead of a plural form of the verb?