Reasons can can vary. Sometimes it's a spammer who is going to converse with themselves about how great a site/service is, that sort of thing. Some people will converse with themselves just to make it look like more people support their opinion on topics in general. Could be someone who wants to "cover" themselves if they ever get banned, so they have an alternate identity already there and ready to use. Could be a variation of the ability to be "invisible" when visiting the forums.

But often it's just someone who forgot they joined another time previously, or they have an old email in their profile and can't retrieve a forgotten password, or maybe they forgot their original username altogether, and it's just easier/faster for them to just create a new account than to ask for help with the problem.

There are probably 1000 other reasons, some reasonable, others more sinister. No way to know unless they tell you really...

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