Is the duel related to this sin?
Yes, but with this difference, that the duel is a thing calculated and is in a sense not fought in the heat of passion; and this circumstance adds to its gravity.
Is the duel, in itself, always essentially bad?
Yes, because therein one jeopardizes one's own life or that of one's neighbour contrary to the law of God, who alone is the master of life.
Next - The Catechism of the Summa - SECOND SECTION - A DETAILED SURVEY OF MAN'S RETURN TO GOD - X. OF THE VICES OPPOSED TO CHARITY AND THEIR ACTS: OF HATRED; DISTASTE FOR SPIRITUAL THINGS AND LAZINESS; ENVY; DISCORD; STRIFE; SCHISM; WAR; QUARRELLING (THE DUEL); SEDITION; SCANDAL (K)