As I understand it, this "unique player" rule is meant to exclude nominating 2 different seasons by the same player
during this nomination round. The player might already have a card or cards in the existing set. I get that from this comment by Garcia:
This means that if two people select one player but different seasons, only the first season will be the one that is on the poll.
In other words, let's say somebody else nominates Rocky Colavito 1959. That rules me out from nominating Colavito 1960. But it's not a problem that there are other Colavito cards already in the set. So far, Garcia has raised no objections to our nominations. You can send a query to <onlinegamesupport@strat-o-matic.com> if you have doubts, though.