Are you looking for the letter itself? It has been translated in The Letters of St. Bernard of Clairvaux, by Bruno Scott James (letter #405, pg. 476-477). Bernard is basically asking Suger to intervene with King Louis to try to prevent the duel. Actually in this case it was a tournament, not exactly a duel. There would have been jousting involved but it was supposed to be for fun, not a vengeance duel.

The church was against all sorts of knightly violence. Tournaments and other violence may have been one of the reasons that the church promoted crusades, to channel this violence into a more worthy cause. Bernard was especially interested in this issue. He was the most active preacher of the Second Crusade, and when the crusade failed it was personally quite humiliating for him, although he tried to deflect blame elsewhere - in this same letter to Suger he complains about knights like Robert and Henry, who (he believes) must not have been in the correct spiritual frame of mind to fight a crusade.

I'm not sure if there are any works about that tournament specifically, but it's mentioned in some other works about the church and violence and the crusades. See:

Richard W. Kaeuper, Chivalry and Violence in Medieval Europe (1999)

Danielle E.A. Park, Papal Protection and the Crusader: Flanders, Champagne, and the Kingdom of France, 1095-1222 (Boydell, 2018)