Check for duplicates only for Person

Because visitor inherits person form = PersonForm makes
visitor fields not visible

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Revert previous commit

Check for duplicates in Person related models.
Just remove Meta.fields from Person form.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>