This page says that Identicons are generated on the basis of hash of the user's IP address.
But, what happens if I change my IP address (for example - log in fro a different machine)?
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Then you get a different icon.
Unless you provide an email. In which case the icon is based on a hash of your email.
The relationship between identicon/Gravatar and email address was clarified for me in this question.
In particular, how come so many 'different' users had the same identicon?