So, I use Arniel's Shade pretty often, as he is almost immune to damage and actually has a pretty smart AI, is free to summon and does decent damage.

An interesting thing happened just now, I was going into Ansilvund but to dispatch of the enemy mage outside the entrance I summoned Arniel's Shade and went invisible just to see how that would go, and Arniel's Shade made pretty quick work of the mage. After that, I went into the dungeon and cleared it entirely, and when I exited I saw Arniel's Shade standing outside where I had originally summoned him.

Bewildered, I walk up to him and low and behold, in the former living Arniel's voice, he goes "Greetings, Arch-mage. Is there anything I can assist you with?"

Then he just starts walking away. His voice is like the living Arniel's, down to every last dialogue of it. With those of you who have the shade, you'd know he sounds dead when he talks normally. Also, his name is "Arniel's Shade" and not "Player's Arniel's Shade".

I'm currently following him to see where he goes, I presume he's going back to College of Winterhold to resume what he normally does, which is quite funny considering he is a ghost but doesn't seem to realize it.

I even summoned Arniel's Shade myself to see if it would make him go away and now there's two of them lol. Anything like this ever happen to anyone before?

Well, Bruce Willis' character didn't know he was a ghost...

Never had that happen to me, but it's sounding a little like the Louis Letrush bug.

I thought of that, but I found it odd that the duplicate was in the exact spot I had summoned him in earlier. I'm following him now to see where he goes, but god does he walk slow.

Searching for this glitch gave me zero results on google/UESP/TES wiki. It seems like a cool glitch if that I wouldn't mind leaving as is as long as it doesn't break my game, that's the all I'm afraid of.

So where did he end up and what did he end up doing?

It ended up getting too late (I followed him for nearly 2 hours of slow walking and got way too bored) and I had to go to sleep, so I fast traveled and called it a night.

I've been looking around college of winterhold for him but I can't seem to find him, so I'm clueless. When I last saw him though, he was heading north from windhelm towards winterhold, so he's gotta be around there somewhere, if he didn't disappear from the game.

I would've paused the game overnight and resumed the next day, but I'm stubborn like that.