You may have noticed that ARS ensures that the proxyAddresses attribute is updated to match the value entered on the general tab (email field). The email field value is not currently checked against the Exchange and can generate duplication of proxyAddresses attribute values.
Product’s design. The proxyAddress attribute’s behaviour described above is a feature in the product, to facilitate the managing of objects from the general tab. It’s true that it currently does not perform any form of validation, which may lead in the duplication of the proxyAddress attribute’s value.
An enhancement request (TF00414390) has been created detailing the feature: when autoupdating proxyAddresses (after email change on General tab) the value being entered should be checked to ensure no duplication occurs.
The product team will evaluate the request and this feature may become available on a future release of the product.