Well you know, if we spend some time looking for a way to prevent multi client, we don't care pretty much about who can still do multi-client... Let's say for example we've got far enough so the only way to have dual client is to use another pc with : proxy, vpn, sandboxie, (+ whatever you think might be used). Chances that someone has enough resources to have dual client are really low. Thus, let's say we got from 80% dual client chance to like 2-3%, it represents like 20/30 people over a thousand players.
Furthermore, the chances that all these players are connected at the same time are pretty low. Thus, dual client is almost impossible and then we can prevent lots of things. Some games did this (I'm thinking mainly about Dofus), where they allowed players to join a solo-client server only (or one where dual is allowed). Thus, even if some managed to bypass the system, it's not enough to ruin people's gameplay.
Anyway, this project is far from having the same budget as a successful MMORPG. Thus, even if we decided to disallow dual client, we might not have enough resources to develop such a great anti-dual system. So instead of working agains't dual clients, it's better to work agains't bots and other things that really ruins a server, which is much more affordable (time relatively speaking).
So, if you have 5 computers and want to open 10000 clients at the same time, just do so. The team here doesn't really wanna play hide and seek with you and try to seek out every player that has multi clients. They have better things to do, so I guess.