Quadtechint.com

While player versus player gameplay is perfectly viable and rewarding,
there is a means of friendlier competition that isn’t as intense as faction
versus faction violence. If you wish to test your skill against other players,
you can challenge them to a duel.
ISSUING A CHALLENGE
A duel challenge can be issued on a PvP or non-PvP server. To challenge a
player to a duel, select the player and then right-click on the player’s
portrait. Select the Duel option. The other player can then either accept or
decline the challenge.
When a challenge is first issued, a duel flag drops down between the two
players, signaling to other players that a duel is about to commence. If the
other player declines the duel, the duel flag disappears. If the other player
accepts, then the duel is on, and the flag continues to wave until a winner
emerges.
As soon as the other player consents to the duel, you and the other player
can begin attacking each other, using the same abilities, spells, and tactics
you employ against the monsters you normally fight against.