Codeigniter Cannot redeclare class Hierarchy

I have some problems with some Hierarchy classes in Codeigniter. I have a 3 level hierarchy on controller:

class Application_controller extends CI_Controller

Another one,

class Newsletter extends Application_controller

and the third one:

class Groups extends Newsletter

I have some constants defined in the classes for configuration to make the code more clean and easy to integrate with other models.

The problem is when i try to access the menu to make a newsletter and send it to some clients i try to do this:

function createNewsletter(){
Groups::TABLE;
}

in that variable i have the table name to get the groups from database. Buit keeps me showing this error:

Cannot redeclare class

I have the require_once in the Classes like usual but it keeps me showing that error. Can you help me?

Thanks in advance,

Elkas

The problem is solved.

I've just being dumb. I was on a infinit loop on the requires.

Example:

require "main.php" in the subclass and in the main class i require "subclass.php", obviously it gives me an error.