Set up this self join relationship on contacts:
Contact::id_account ≠ DupEmailsContact::id_account AND
Contact::Email = DupEmailsContact::Email
DupEmailsContact is a second occurrence of Contact.
If you have only one contact record for each account the first pair of fields are not needed. It's used to omit contact records from the same account.
Then this calculation: Not IsEmpty ( DupEmailsContact::id_account ) will be true if you have a duplicate email for a contact record from a different account.
You can use this with a conditional format to make layout text "Duplicate Email in ID_Account" visible on your layout or you can put it in a calculation field like this:
If ( Not IsEmpty ( DupEmailsContact::id_account ) ; "Duplicate Email in ID_Account" )
If id_account is of type number you can simplify it to:
If ( DupEmailsContact::id_account ; "Duplicate Email in ID_Account" )
Works like a charm.....
Thanx again PhilMod...