A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 13 April 2010
Two questions concerning Mirumoto Bokusui.
A. If I tie a duel, is my Mirumoto Bokusui destroyed?
B. If one of my other personalities loses a lethal duel, for example a Steel on Steel, do I get to play Duty with my Bokusui before he is destroyed?
Relevant texts:
Mirumoto Bokusui Dragon Clan • Samurai • Duelist After one of your Personalities loses a duel, or if another player ever controls more Rings than you: Destroy Bokusui.
Duty Bushido Virtue Reaction: Before another player’s card’s effect destroys one of your Personalities, destroy your target honorable Samurai Personality at the same location: Negate the first Personality’s destruction.
Steel on Steel Iaijutsu Battle: Target your unbowed Personality: He challenges a target enemy Personality. Destroy the duel's loser.
Brook Cunningham (rules official), 13 April 2010
A. If I tie a duel, is my Mirumoto Bokusui destroyed?
Yes. Having tied totals in a duel results in both Personalities "losing".
B. If one of my other personalities loses a lethal duel, for example a Steel on Steel, do I get to play Duty with my Bokusui before he is destroyed?
No.
A lethal duel's loser must be deterimed first. Then, after that, he is destroyed. But immediately after the determination of the loser, Bokusei's trait destroys him. Therefore, he is no longer in play at the timing point "Before another player’s card’s effect destroys one of your Personalities", which would come just after that.