app/models/Data/User.php: add uniqueness validation for username and email.

app/models/Data/User.php

@@ -3,6 +3,8 @@
 namespace App\Model\Data;
 
 use Phalcon\Mvc\Model,
+    Phalcon\Validation,
+    Phalcon\Validation\Validator\Uniqueness,
     InvalidArgumentException,
     Httpcb\OAuth\UserData\UserDataInterface;
 
@@ -39,6 +41,17 @@ class User extends Model
         $this->setEventsManager($this->getDI()->get('eventsManager'));
     }
 
+    public function validation()
+    {
+        // Validation
+        $validator = new Validation();
+
+        $validator->add('username', new Uniqueness(['message' => 'The username already exists.']));
+        $validator->add('email', new Uniqueness(['message' => 'The email address already exists.']));
+
+        return $this->validate($validator);
+    }
+
     /**
      * @return mixed
      */