This is no longer a part of the World of Warcraft API. Use StartDuelUnit instead. It was removed in patch (unknown, very early).

Invites the specified player to a duel.

Duel(playerName);

Arguments

(String playerName)

playerName 

The name of the player you wish to duel

Returns

nil

Example

Duel("Sembiance");

Result

Invite Sembiance to duel you.