JSF Validation using DAO doesn't work - validation

Question

I'm trying to validate the information, user is giving at the registration. One of the fields contains the mailadress, which should be validated by looking in the database to confirm, it doesn't exist yet.
Problem is, that if i type an existing mailadress, it will give back a NonUniqueResultException, but also does store the new user with the duplicate mailadress in the database. Don't understand this, beacuse in the JSF-lifecycle after validation fails, it shouldn't go on to the invoke application phase, right?
Here's my code:
mail field in register formular
<b:inputText id="mail" required="true"
requiredMessage="Bitte geben Sie Ihre E-Mail-Adresse an!"
label="E-Mail" placeholder="name#example.com" value="#{registrierenManagedBean.nutzer.mail}">
<f:validator validatorId="mailValidatorRegistrieren"/>
<b:messages for="mail"/>
</b:inputText>
my custom validator
#FacesValidator("mailValidatorRegistrieren")
public class MailValidatorRegistrieren implements Validator {
#EJB
private DAO dao;
private String mail;
private static final Pattern EMAIL_PATTERN =
Pattern.compile("^[A-Z0-9._%+-]+#[A-Z0-9.-]+\\.[A-Z]{2,6}$", Pattern.CASE_INSENSITIVE);
#Override
public void validate(FacesContext facesContext, UIComponent uiComponent, Object o) throws ValidatorException {
mail = (String)o;
boolean matchesPattern = EMAIL_PATTERN.matcher(mail).find();
if(!matchesPattern)
{
throw new ValidatorException((new FacesMessage("Invalid mail")));
}
if(mail.isEmpty()) {
return;
} else if(validateNutzer(mail)){
throw new ValidatorException(new FacesMessage("mail alredy used"));
} else{
return;
}
}
private boolean validateNutzer(String mail) {
try {
Nutzer n = dao.findNutzerByMail(mail);
return n.getMail().equals(mail);
} catch (NullPointerException e) {
return false;
}
}
}
and the "findNutzerByMail"-method from my DAO
public Nutzer findNutzerByMail(String mail) {
try {
return em.createNamedQuery("findNutzerByMail", Nutzer.class)
.setParameter("mail", mail)
.getSingleResult();
} catch (NoResultException e) {
return null;
}
}