They're probably referring to the fact that the Cherokee members are already represented by their US legislators in the same way that other US residents are. Providing them this vote would give them more representation, and they're asking if you believe that's a better solution.

Would it? I think it's possible that other laws may mean that they have one vote, and thus have to choose whether to have their vote apply in the congressional district they are physically in, or to the Cherokee representative seat they are alotted for a non-geographical district. That would retain one vote for one person. This isn't even all that weird, I assume. Do people with residences in multiple districts get to vote in each, or do they have to choose?

They definitely have to choose, and many places I've lived in explicitly mention that also voting in another place (in the US) is a crime.

Relevantly - if their Cherokee delegate is a non-voting delegate, then while they may have two representatives on the floor of the house speaking for their interests, they only have one who can be involved in the formal exercise of said House's power.