Learn Italian: La spiaggia di Venezia è la più affollata e la più sporca. - The beach in Venice is the most crowded and the dirtiest.

Adding the definite article (il, la, i, or le) before più turns a comparison into a "relative superlative" (meaning the most). Without the la, più affollata just means "more crowded." By saying la più affollata, you are saying it is "the most crowded."

Do I really need to repeat la più before the second adjective, sporca?

Yes, it is usually best to repeat both the article (la) and più for each adjective in a list. While in English you might shorten it to "the most crowded and dirtiest," Italian prefers to be explicit: la più affollata e la più sporca (literally, "the most crowded and the most dirty").

Why do the adjectives affollata and sporca both end in -a here?

Even though they are part of a superlative phrase, they are still just adjectives describing a noun. They must agree in gender and number with la spiaggia (the beach), which is feminine and singular. That is why the base words affollato and sporco change to end in -a.

Why does the sentence use di Venezia to say "in Venice"?