Secure embedding externalIdentifier

I was trying to use secure embedding by providing my current user email address in externalIdentifier but it goes and create a new user with the same email address.
When i tried to include the email address in userInfo.email it gives an error saying

    "error": true,
    "message": "Validation error: email must be unique, organizationId must be unique"

May i know correct combination to not create new users.

In additionally i have few metadata i hope this won't be an issue...

And what's the difference between Product experience in Embed user Desktop + mobile ?
Which cost structure apply for Embed user?

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Any update on this would be really appreciated....