PowerShell - New mailcontact - two objects cannot have the same proxy address

Hello,

I import contacts via this command:

import-csv c:\import4.csv -Delimiter ';' | foreach { new-mailcontact -alias $_.Alias -name $_.DisplayName -ExternalEmailAddress $_.EmailAddress -OrganizationalUnit “domaine.com/architecture/utilisateurs/Contacts”}

My problem is that some contacts have the same email address.
How creating an email address even if it already exists?

Thank You !