[php] "Cannot send session cache limiter - headers already sent"

Having a problem with sessions which is becoming very annoying. Every time I try to start a session on a particular page I get the following error:

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at ............ on line 23

using this code:

<?php
session_start();
if(isset($_SESSION['user']))
    {
        $user = $_SESSION['user'];
        echo "$user";
    }
else
    {
    }
?> 

Is it suggesting I've already used session_start(); ? Ive had a look around but nothing really clears this up for me.

Thanks