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);

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Arguments

(String playerName)

playerName 

The name of the player you wish to duel

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Returns

nil

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Example

Duel("Sembiance");

Result

Invite Sembiance to duel you.