disallow presence of duplicate at symbol

done for [AGPUSH-976](https://issues.jboss.org/browse/AGPUSH-976)

although there is a [whole science](http://www.ex-parrot.com/~pdw/Mail-RFC822-Address.html) on what constitutes a valid email address, let's not take it to the extreme, this PR helps to take care most of the valid cases.