I wonder if a policy allows a user register several accounts in stack exchange? Isn't it treated as a reputation cheating mechanism?
A user registering multiple accounts is particularly discouraged. However, some people may accidentally create multiple accounts, and those accounts can be merged when identified.
If you feel someone has abusing multiple accounts for nefarious purposes (like reputation sockpuppeting) please flag the posts with a custom flag and explain as much as possible, preferably with links if you've identified the culprits. From there, we can leverage mod tools to confirm the abuse and take appropriate action.