Mkay, now I'm serious. With strong historical evidence pointing to Kunihiro ( possibly ) being a duplicate and/or fictional blade, what are Izuminokami's thoughts on the matter? Is he aware of the fact?
First of all, how dare you poison my water, destroy my crops and deliver a plague to my house? Disowned.
Let’s go with Horikawa being a duplicate. Taking into account one of his kiwame quotes; Izuminokami knows of nothing before (and/or maybe after) the Bakumatsu Period, so we could assume it’s safe to say that he doesn’t know much about Horikawa blades beyond their reputation for being masterpieces. (Yamanbagiri’s constant griping aside) To him, Horikawa has always been his partner and the reason for his existence. He was forged to be a partner blade to Horikawa Kunihiro. Without Hori, Izuminokami would not exist. In this world or the world of Hijikata Toshizou.
Establishing this fact, we come to the possibility that Horikawa Kunihiro is possible a duplicate blade, much like Yamanbagiri. And let’s say Izuminokami finds this out now. His thoughts?
Quite honestly, I think Izuminokami would not care.
Why? First off, let’s go with the obvious. If a duplicate blade was good enough for the vice-captain of the Shinsengumi, then in Izuminokami’s mind, that’s good enough for him. (Granted, this is only coming to light as of now, but surely they might have been rumors?) Duplicate or not, the fact remains that Izuminokami is a blade built in the likeness of Horikawa. That is the only reason he exists, beyond Hijikata wanting a Kanesada sword.
But on a personal level for Izuminokami? Horikawa has been his partner for centuries. They fought together, laughed together, bled together and were together until the fateful battle when Hijikata sent him away. As far as Izuminokami is concerned, it doesn’t matter whether Horikawa is a fake. He is the only Horikawa Kunihiro he’s ever known. He’s his partner, his friend and his comrade. No pedigree or authenticity will ever change that. Horikawa has proven time and again to be a capable blade/warrior, which matches to Izuminokami’s memories of him from all those years ago; proving that a possible duplicate is just as capable as a genuine article.
To conclude: Izuminokami will always see Horikawa as who he is, rather than what he is. His faithful and dependable partner and friend.