No error message when trying to create a FO administrator, who is already a FO user.

[PBD-ILR-LU]

A Monarc FO instance administrator left his company. He had to be replaced using the BO. The new e-mail address has been configured. The issue was that this new e-mail address is the one of a person already defined as user of the FO. So whereas no change has been done on FO side, the BO has not displayed any error message. It would be important that an error message would be displayed.