All the accounts a user controls will be treated as the same person. In short, registering another username does not allow someone to create a new identity for themself: it won't allow a user to circumvent a block or vote more than once.
Sock Puppets[]
An undisclosed alternate account used for disruptive or deceptive purposes is known as a sock puppet. Sockpuppets are inherently illegal and any user found posessing one may be banned given sufficient evidence. Please mark all known sock puppets with the {{Sock puppet}} template.
Alternate Personas[]
Given the right circumstances alternate personas are tolerated, though on an exceedingly rare basis. An alternate persona is a sockpuppet that has been openly declared to be such and will then become known as a Doppelganger. Doppelganger creation must be approved by the administration, otherwise, regardless of name or intentions, it will be considered a sockpuppet and appropriate authoritative measures will be taken to ensure its removal.