smarthost Exchange 2016

in the following scenario: DAG 2 Exchange server 2016 as smart host on external mailserver (register.it) all mailboxes regularly belong to users of the domain, except one which is consulted only on the external mailserver (register.it) and the user does not have a mailbox in the DAG.

if I send an email from an external domain (for example @gmail.com) to the disconnected mailbox AND to another regular mailbox , it arrives duplicated, from the external SMTP and from the internal one. How can I solve the problem?