This bug is on lichess!

@Eror404 I'm sorry, I don't read this language, and I don't know which page this has been seen on. Can you provide a link to where you have seen this, and more information in English?

Ну так скобочка это не буква, не цифры и не подчёркивание. Поэтому такой ник зарегать нельзя. Можно только А0_00

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You cannot use brackets in sign up username
it is a rule of lichess and playstrategy as well as all other forks.

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So the bug is that this message is displayed twice? Or something else?

yes that's why it's a small mistake

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