Short answer
That's against the Community Guidelines, whether they do it by cross-checking public information, by technical tricks, or by asking around. Report the profile or the posts (see “How do I report a post, message or profile?”), and if it's happening across messages, email [email protected] with screenshots.
Nixyum never confirms or denies who owns a Nyx to anyone who asks, including other users claiming they "already know". See “How do I keep my Nyx separate from my Nex?” for ways to reduce the risk.
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