Duels - Sword Art Online

Duels (デュエル dyueru) are a functionality that allows two players to check their damage dealing rate (DPS) or settle a heavy score, optionally disabling the safe area effect by mutual agreement when both players accept the prompt, at the same time not changing the player's status color from green.

This functionality is confirmed to exist in the VRMMORPGs Sword Art Online and ALfheim Online.

Mechanics

A duel is initiated via the user interface in which the challenging player sends a request to whomever they want to duel. It pops up as, "X has challenged you to a one-on-one duel. Do you accept? Yes/No" with the radiobuttons for selecting the victory condition. There the challenge can be accepted or denied, and in the case the request is accepted, the accepting player is the one to choose the dueling terms. The system message for the duel the pops up above the duelists, and the 60-second countdown to the battle begins, during which time the both players are to make preparations. Following that, the Anti-criminal Code becomes ineffective for the duelists for the duration of the fight.

If after the time limit both players fail to achieve the victory condition for the duel, the victory is given to the one who has more hit points left. If the difference between the hit points of both players is less than five, a draw is declared.

In the web novel version of SAO, the Starting City had a fighting arena during beta test stage where it was possible to initiate duels without the other player's consent.

The 60-second countdown received negative feedback in beta stage as being too long.

Victory Conditions

There are three kinds of victory conditions that can be chosen between the players.

In the First-Strike Mode, the duel is decided with the first hit landed. Additionally, block damage also counts if it accumulates to half the opponent's hit points.
In the Half-Damage Mode (or Half-Loss, or Half-Finish), the duel ends when one of the players loses half of their hit points.
In the Total Loss Mode (or Full-Finish), the duel ends with a player's death.

It makes sense that very few would choose the Total Loss Mode willingly in the game where death is permanent, including the player's real body. However, the duel system can be exploitable in SAO, and usage for sleeping player kills (Sleep PK) has been recorded.
Interestingly, even if the players go with the Half-Damage Mode, the HP may easily fall way below half if the finisher is a critical hit, and so the largest portion of the duels are the one-hit First Strike ones.

Known Duels

References

Duels - Fan Wiki
Anti-criminal Code Effect Area - Fan Wiki
Sword Art Online Progressive Light Novel (Yen Press)