MUSASHI DID NOT KILL KOJIRO-PART 2 It can be instructive to select such a famous historic figure as Miyamoto Musashi and study both the facts and the folklore surrounding this swordsman. Doing this particular to Musashi is nothing new but I think we’ve found some new general information that brings a bit of clarity to the questions if not the answers…
Transcending geographical borders, the name “Miyamoto Musashi” is known all over the world. Much of this common “knowledge” can be attributed to some very enjoyable works of fiction that have been translated into English, most notably, Yoshikawa, Eiji’s “Musashi” as well as non-fiction (depending upon the translator) works such as Musashi’s own text on heiho (strategy) called “Go Rin no Sho” (Book of Five Rings) and various publications that emerged after his death that are more suspect.
We discussed an article made available from the Hosokawa clan treasure trove. This information dovetails that article nicely.
info Written in Nuumatakaki ( Numata Engen's family diary)
Regarding the duel with Kojiro, Sasaki:
Both men were at the top of their games and both had perfected their own winning heiho (strategy) for sword fighting. Both men had many deshii (students) and regional if not national fame. (See shijyo).
It was agreed beforehand that the two duelists would meet on Ganryushima  without deshii (students or followers). Kojiro kept this promise. Musashi did not. Musashi’s students mingled in with the spectators to watch the duel and then took matters into their own hands by completing the grisly task of dispatching Kojiro as he roused from being dazed by being struck on the head by Musashi.
Fear of retribution forced the famous Musashi into hiding deep within the Hosokawa fief’s castle town of Kumamoto and under the protection of the garrison’s guns. Musashi was forever grateful and was hired with a stipend of 300 koku and later retired within the Hosokawa domains.
 Named for Kojiro’s’ style of swordsmanship after his assassination on the island by Musashi’s students.