Hi

In fact "Vous" here is a "politeness form", isn't it? it is addressed to a single person.

In this case you don't grant the past participle.

Here is a link which explains it : http://www.aidenet.eu/grammaire12a1.htm

You can also compare the results in google between "Vous êtes déjà identifiés" and "Vous êtes déjà identifié" and you can see the difference of results largely in favor of "Vous êtes déjà identifié"