When did the last formal and recorded duel take place?
It seems that duels are no longer fought because of a combination of both changes in law and in fashions. E.g. Tom Reiss‘ The Black Count: Glory, Revolution, Betrayal, and the Real Count of Monte Cristo relates that duels were illegal in Paris already by approx. 1805, yet the practice was still fairly common at the time to settle serious disputes among “men of honor”.