It appears that posting online comments under multiple aliases is against the law. Well at least it is for one man. Raphael Haim Golb, 49, was arrested for supposedly using over 50 different email addresses and monikers.
Golb wanted to bloster his arguments about the orgins of the Dead Sea Scrolls so he used some names belonging to acedemic rivals.
Golb allegedly used multiple online sock puppets or fake identities to make it seem like he had followers.
Golb is also suspected of publishing a false confession to plagiarism under the name of Professor Lawrence Schiffman, the chairman of Judaic studies at New York University who had been critical of Golb’s theories
Prosecutors claim that Golb’s false identities allowed him to obtain a benefit, meaning they can charge him with criminal impersonation, identity theft and aggravated harassment. But Golb’s lawyer, Ron Kuby, told the Times that “gaining an advantage in academic debate about the Dead Sea Scrolls is not the kind of benefit required by the law.”
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I dont think theres any specific law against it. from my understanding of the story the problem was he allegedly created an alias misrepresenting himself as someone else in order to mislead others and damage the character of this other person. obviously there's a lot more to it, but i wouldn't worry if you went to dozens of muppets forums and posted as miss piggy, kermit the frog, and a dozen other muppet aliases.
As long as you're not doing it to harass or attack someone.