Question: How does a Stack Overflow moderator know about sock puppet accounts? 

How do they trap the user?

Answer: 
You know, throughout your posts here, you keep referring to this as a "bug" or a "trap". It's not a bug at all; it's a *feature* designed to prevent people from abusing the site. **Which is exactly what you were doing.**

In your comments on Marc's answer, you complain that your friend "Nahid" got her account merged due to suspicious behavior. If that's true, and Nahid actually exists and was playing by the rules, then what happened to her was because *your behavior* made her look like another one of your sock puppets.

Why did this happen? Not because of "bugs" in Stack Overflow, or the malice of the evil moderators. It happened because you abused this site *so badly* that everyone around you was thrown into suspicion. If you hadn't tried so hard to make your fake accounts look like real ones, your friend's real account wouldn't have looked like another of your fakes. If "Nahid" is upset that her account got merged, she should be blaming you for that. If you're upset that you lost reputation, you should blame yourself. **You cheated. You got caught cheating. That's your fault, not Stack Overflow's.**

You've suggested that SO might lose users due to features like this, but I think it'll *gain* users when they see that their reputation means something. People don't like being taken advantage of by others who don't play by the rules, and they don't like cheaters filling up their community space with garbage. Even in your imaginary "99 lost professional users" scenario, I suspect the number we'll gain far outweighs the number we'll lose due to accidental merges.

To sum up, the solution isn't to ask how to cheat more effectively, and it's also not to blame others for noticing. The solution is to improve your behavior, start playing by the rules, and try to be valuable to the rest of this community.
Actions speak louder than IP address… Possibly even a “view” of a question is an action.