User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-26209278-20150613141955/@comment-26007105-20150615133411

I'm assuming that by doppelganger, you're talking about the second Issei, in which case, yes, we do know, at least to some degree. The other Issei was a manifestation from the scale mail shard that the Old Satan Faction found, tainted by Loki's curse, so the other Issei and the shard inside Rias that formed her Scale Mail are the same thing. Well, that's how I see it at least, I may be wrong.