why the system does not allow repetition of the mail in different accounts. In my case I have many real estate agencies that pay different services.
And I have to send the bill of different house to those agencies
you want to use login to customer portal as email, and want to use more than one email to different customers.
it’s completely wrong strategy 
use login for that case …
Config / Main / Portal → Authentication field set to Login
please note, from this change all your customers can login only with login, no with email
I think he means using the same email for multiple accounts. I have a company that has 2 restaurants. Both locations are different accounts but they are owned by 1 email address. It would not let me add the same email to 2 different accounts.
Possible solution: one account in Splynx, and multiple services (Customer / Services / Internet)
That would work as a work around, but you would only have 1 service address recorded. I would prefer to have a seperate account for each service address.
Yes, I understand, thanks Ruslan and spencer
You can always use description field in service and type in connection address there. that way you would have all connection adresses on same invoice.