uniquenessvalidationxpath

I have the validation rule that works fine when the user tries to change the current email address to another email address.

But if the current one is blank and they enter a duplicate one, the portal accepts the duplicate value. Please help.

<my:Property my:Name="UniquenessValidationXPath" my:Value="/Person[not(ObjectID='%ObjectID%') and ExternalEmail='%VALUE%']"/>