Hi @Petja Belemezova ,
Thanks for reaching out.
I understand that you are looking to restrict duplicate entries in Azure B2C while sign up.
As of now, Azure AD B2C User Flow does not have policy to customize built-in Sign up flow or restrict duplicate entries.
However, Azure AD B2C allow to use API connectors to customize the sign up with REST APIs which can be invoked before creating the user to validate the email address and restrict the duplicate user to Sign up with same email address.
There is detailed article to use API connector to check duplicate email which you can enable before creating the user :
https://dev.to/luperi/azure-ad-b2c-preventing-duplicate-emails-with-multiple-idens-3c5k
Hope this will helps.
Thanks,
Shweta
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