If you are participating in this behavior, please stop. If you actively maintain more than one account, they are all at risk for deletion, or at the very least vote invalidation.
The below Meta post is written by one such Community Manager, and he lays out the reasons this behavior is problematic and when it is acceptable (just don't expect duplicate accounts to hang around). While that post does recommend moderators simply suspend the users involved, the CM team seems to have hardened their stance and they are now advising me to delete sock puppets.
Please read How should sockpuppets be handled on Stack Exchange? Of note:
When should sockpuppets be considered a problem?
There are a handful of legitimate reasons to maintain multiple accounts. A good rule of thumb for identifying abusive socks is: if the second account allows you to do something on the site that your normal account would be prevented from doing, it is abuse. Examples of this include (but are not limited to):
- Voting on your own posts
- Answering your own questions with the other account(s)
- Casting multiple votes on others' posts
- Supporting your own arguments ("+1: shog is right, don't know why the rest of you don't realize this")
- Using bounties to circumvent the rep cap
- Circumventing suspensions, quality bans, or the rate limits on posting questions / answers / comments / etc.
You mentioned valid uses for sockpuppets; what are they?
I've probably had a half dozen or so alternate accounts over the course of the site. They're useful sometimes: testing bugs that only show up at low rep levels, reminding yourself what the site "feels like" for someone with only the basic abilities, maintaining a highly unnatural q/a ratio... That said, I always treat these as disposable accounts, and some of them have been removed (either automatically or via mod intervention) when I crossed a line somewhere - you should never assume that a sockpuppet is "protected" if you're actively using multiple accounts on one site; it's entirely too hard to be certain you're not interacting in some inappropriate fashion.