I know users can have more than one account on any given site; as long as they don't use their 2nd or 3rd accounts as sockpuppets, Stack Exchange is happy. There's a whole series of argumentations that justify this sort of behaviour, so I'm not concerned here with its ethics.
But what about voting?
Can a user with two or more accounts upvote or downvote someone else's post more than once?
Can a user with multiple accounts upvote their own comments?
These seem rhetorical questions but this type of behaviour is quite different from serial downvoting and also much harder to detect.
To clarify, they are not upvoting their own posts. Instead, they are using their secret accounts to up/downvote someone else's post multiple times.
I can't prove that the upvotes on comments are cast by the same user, but if I become aware that they have at least two accounts I may strongly suspect malicious behavior, e.g. drumming support for their comments and driving upvotes on specific questions they have chosen to answer. I'd like to know for certain if this is a flaggable offense.