Does a monopoly in a commodity in Sheriff of Nottingham gain both bonuses?
As per the rules (may need a BoardGameGeek.com account to access):


  The player who has successfully delivered the most and the second most of each type of Legal Good is declared the “King” and “Queen”, respectively, of that Good.


I would interpret this to mean each player can either be the King or Queen (or split the points in a tie), but not both. Hence, even a monopoly on a Good would only net the King's bonus.

If the single commodity holder got both the King and Queen bonuses, you could also make the following argument: when there isn't a monopoly, the player with the highest number of goods also always ties for the second highest number of goods. That player would always get the King's points and some portion of the Queen's. This doesn't seem like a logical interpretation of the rules, and most people with the second most number of Goods would be pretty annoyed if this were happening. If a player only gets a single bonus when there isn't a monopoly, the same rule should apply unless otherwise stated by the rulebook, which in this case it isn't.