[Matt]

If you're doing the repeatable Field Duty quest from Cenarion Hold, you can complete it twice per completion of the event, but you have to be quick. I believe you get 40 reputation total instead of 20.

Get the quest, go to the camp and start the event where your group and the NPCs have to fight the bug boss. Once the bug is killed (and the captain lived), mount up right away (epic mount) and equip your Mithril Spurs, Riding Gloves, and Carrot on a Stick (you don't need all 3 but it helps). Immediately when the captain will take your field duty papers, complete that, and run back to Cenarion Hold as fast as possible. Go back to the NPC, complete the quest, and pick up Field Duty again. (Here's a great mod to speed up the quest completion: TurnIn.) While still mounted run back and if you are fast enough, the captain will still be there and you can complete the quest again without doing the event again.

You have to be very fast so it may take some practice. Attached is a screenshot of the general path I like to take in both directions. Travel just south of the enemy camp so the mobs don't aggro you and dismount you.