Model Scenario Problem

Hi,
Im trying do make a ChangePassword Scenario using Users Model, but Im getting that error when trying to validade:

[username] => Array ( [0] => Username "admin" already exists. )

At Change Password view only have CurrentPassword, NewPassword and cNewPassword fields.


User.php (Model):

...
	public function rules() {
		return array(
		
			array('name, lastname, email, gender, birthday, password, username', 'required', 'on'=>'register'),
			
			array('username, email', 'unique'),
			
			array('currentpassword', 'required', 'on'=>'changepassword'),
			array('newpassword', 'required', 'on'=>'changepassword'),
			array('cnewpassword', 'compare', 'on'=>'changepassword', 'compareAttribute' => 'newpassword'),
			array('newpassword', 'length', 'min' => 5, 'max' => 20),
		);
	}
...

UserController.php:

...
	public function actionChangePassword()
	{		
		$model = new User('changepassword');
	
		if(isset($_POST['User']))
		{
			$model->attributes=$_POST['User'];

			if($model->validate('changePassword')) {
				$model=User::model()->findByPk(Yii::app()->user->id);

				if(md5($_POST['User']['currentpassword'])!=trim($model->password)) {
					Yii::app()->user->setFlash('error', "Incorrect current password..");
					$this->refresh();
				}

				$model->setAttributes($_POST['User']);
				$model->password=md5($_POST['User']['newpassword']);

				if ($model->save()) {
					Yii::app()->user->setFlash('success', "Password Changed!");
					$this->refresh();
				}
				
			}
		}
		$this->render('changepassword', array('model'=> $model) );
	}
...