Is Kashin Koji actually someone who has appeared in Naruto before?
Sure, there are coincidences that Kashin Koji and Jiraiya could be the same, but you also have to realize that Jiraiya died and there was no way he was surviving after the fight with Pain.

Pain finished him and that was the end of Jiraiya, now where the Rasengan and the toad summoning comes into place, it could be very possible that he might have been a relative of Jiraya, but as it looks, we won't find out until we get more information.