You really need to identify the specific behaviors that you want to prevent.
For example, if the real problem that you want to prevent is not "people making multiple accounts", but "individuals being targeted by multiple accounts controlled by the same person", that's an area where you can use similar traits between potentially clone accounts to allow the players themselves to police multiplaying. In other words, if they detect traits of multiplaying, they can act (somewherevia some interface) to check for similarities between the accounts, and if the accounts do have too many similarities, cause a punishment to occur. Player policing. The benefit of this is that multi-accounting matters mostmatters the most to other, non-multiplaying individuals, so if they feel wronged, they have a recoursethey have a recourse they can try to catch the multiplayer on the spot. It also allows things that in a flatly viewed account system would be false positives (like ip) to be much more real evidence of multiaccounting.
Similarly, as others have mentioned, if it's the gaining of rank by defeating their own dummy accounts, a system like the rep system here on stackexchange helps to turn taking ranking points from yourself into a zero-sum gain.