and I needed to pickpocket the real one for a Thieves Guild mission. Problem is since he magically spawned duplicates I can't even get close to him without being detected. I even tried becoming invisible to pickpocket but it didn't work! Anyone help me on how to complete this mission?

Here's a vid of it if you don't know what I'm talking about.

http://www.youtube.com/watch?v=M46QWYvReEY

If its a side job from vex or delvin, you can just go back and tell them you want a different job. You could also just pick his pocket while youre detected, your chances of success are lower, but its still possible.

But it will say I failed the quest which I don't want to do for personal reasons.

Fair enough, see my edit above then.

I've tried and tried that way but because there are more than one, I keep failing x.x'

If youre on pc you could force the quest to complete with the console command.

Sadly I'm not lol. Would have done that first thing.

Try casting invisibility or drinking a potion of the same to get up close without being noticed.

or a potion of pickpocketing?

tried that didn't work

haven't tried this but I doubt it will work

http://cloud.steampowered.com/ugc/594715184702210940/6B0C716C814B99AD77F70B7D4D369A9D3546E796/. Happy without clones. Who'd want to clone him any way. I think the glitch occurs when you try to kill him, while he is standing outside Whiterun. He got killed in Riften in one of my playthroughs and he disapeared just fine.

lucky