I cannot add one of my customers with the same e-mail address extension to the project.

[Emine Nur Yılmaz]

I added 2 of my customers with the same e-mail address extension to the project, but I cannot add 1 customer. 

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(You do not have permission to invite customer to support project). 

Please check the email addresses and try again.

I receive the warnings above, but I can add different people with the same e-mail address extension.