When Musashi shows up to find his impostor, he will have no choice but to simply duel with Seijuurou. Gion is counting on Seijuurou's wish to prove his might as a swordsman.
After the duel, Ueda and his men will kill the winner. At the same time, however, Gion will have to take care of the other brother.
And so, Denshichirou is cornered at the Sanjuusangendou, a Tendai temple in the Kyoto region. Musashi shows up and does battle with Gion, who tries to stop him, telling him that if he just leaves, nobody will ever need to find out his secret identity, but, if he killed him, there would be no way to escape everyone finding out.
Musashi chooses to drop his Kiryuu persona, at least for the time being, and to use his skill to protect those in need. So, he defeats Gion. Although he killed Denshichirou's older brother in a duel and is a wanted criminal, Denshichirou claps and praises Musashi.
This is another man who is mentioned in a few histories and in retellings in popular fiction such as Yoshikawa Eiji's book, but, little is known about him because his name does not exactly match others on record. He may have been Yoshioka Naoshige. This is not entirely unheard of, as people used to have many different names in days of old.