"Does someone have a better one?"
Translation:Hat jemand eine bessere?
I am afraid not. Gender and case are two different things. The forms may look like other forms but they are still two different categories. The gender is a characteristic of the noun. Verbs determine the case. "haben" requires a nominative subject and an accusative object. "Ich habe eine Katze/einen Hund/ein Pferd." In the given sentence "Hat jemand eine bessere?" we do not know what is better but the gender is feminine. "ein besseres" = neuter, "einen besseren" = masculine
Thanks! I think my mistake initially was that I had written ein bessere, and Duolingo said that the correct translation was eine bessere. From what you are saying, I guess that there should be 3 correct answers, Hat jemand eine bessere, Hat jemand ein besseres and Hat jemand einen besseren.