This is actually pretty old, there is also an easy macro for this since most players know the trick of not dueling with someone flagged. Here is the way around it. First, Type in /macro press enter. Your macro interface will now appear. Create a new macro & give it a name, I call mine "Flag Alliance" now give it an icon preferably one your class does not use I personally just use the ? icon since it stands out. Now for the body of the macro itself.
/pvp
/forfeit or /concede (your choice)
/pvp
Not being in the game I honestly can't remember if there is a Save button you have to press first or not, I usually don't press anything, & just move the icon to my hotbar somewhere for easy access.
Effectively this allows you to begin a duel while not being flagged, then while they are beating on you, or are just about to finish a cast use it. This will instantaniously flag you, exit the duel, & then turn pvp off so that after 5mins it will disappear. Great for those rare occasions when a 70 is nearby or if the player you just killed was an alt of a 70.Have fun with this macro, which of course is an old one as well. Hopefully this will help you kill them easier, & if you like my little post here please give me some rep. :P