I recently started a new game. There is an invisible copy (or something) of my character lurking around. First there were subtle hints. I would start to walk, but I wouldn't go anywhere immediately, like I was trying to walk against something but nothing was there. I thought it was just glitchyness with the nav mesh. Then, came the first time I saw it, sort of. I'd just shot a bandit in Embershard mine. I realized I was still targeting someone. Someone named "You". It went away that time. Turned up again in Reachcliff cave while I was fighting a draugr. I hit YOU with an arrow. It was bobbing around in the air. You apparently summoned an atronach too. I know I didn't do it.

All the NPCs can see You, as You just made the Forsworn at Sundered Towers go berserk. It was a hoot to watch them fighting the air and ignoring me while I picked them off. Unfortunately, You has a hostile attitude towards me, but other than yelling threats they haven't done anything to me.

Could this be some major error with the decoy perk? I don't actually have that perk yet, but this is kinda behaving as described

I never heard of this before. Probably it's caused by a mod and i'm very curious what's the name of that mod and if you can fight your doppleganger. It would be interesting to fight someone with exact same equipment, skill levels, perks...

There's a spell in either Apocalypse Magic or Lost Grimoire that has a spell with a similar effect. It creates temporary twin clones of yourself, but it's buggy as hell. The first problem is that if you hit one of them by accident, or if one of them uses an AoE spell too close to the other one the game will treat it as if you attacked a member of whatever guilds you belong to. The second problem is that they can use the Storm Call shout and it doesn't go away until you either leave the area (like from Whiterun to the other side of Ft. Greymoor), or go indoors. Clear Skies will not work against it (haven't tried the Control Weather spell as I forgot I had picked it up when this occurred). The last problem is that they either attempt to use the same spell, or something else that I have and it causes the game to CTD.

Are you secretly Ralis Sedarys? You sound like Ralis....

Nobody give this person a pickaxe or any miners, it's a trap

Hmmm. Do you have http://www.nexusmods.com/skyrim/mods/50349/?installed perhaps with one of the other mods mentioned? It changes dialogs and messages to refer to "you" and so on....