All players /roll 5 (or some low number).

There are no tie-breaker rounds. In the case of multiple winners, all winners will split the winnings.

Winner: The player(s) who rolled the 'rarest' number (ie. the number that was rolled the least)

Loser: ALL player(s) who rolled the least rare number (ie. the number that was rolled the most)

Payment Amount: The wager amount.

If something silly happens, like everyone rolls the same number, nobody rolls the same number, or the rarest number is equally as rare as the least rare number, the round should result in a draw.

Alternative Idea: Cursed Number

The dealer automatically performs a /roll 5 (or some low number) to determine the cursed number. All players then /roll 5.

If nobody rolled the cursed number, an additional round of rolling is done until somebody does.
In the case of multiple losers, each loser must pay the winners the full amount.

Winner: ALL players who did not roll the cursed number

Loser: The player(s) who rolled the cursed number

Payment Amount: The wager amount, equally split among the winners

Maybe better idea for Cursed Number:

The dealer automatically performs a /roll 10 (with 10 counting as 0) to determine the cursed number

All players /roll 999999 (highest roll possible)

Winner: The player who rolled the least cursed digits

Loser: The player who rolled the most cursed digits

Payment Amount: The wager amount

Tiebreaker rounds can resolve ties.