FPC Duplicating

when I jump to new Scene level thru portal and than I return back on the previus Scene my FPC gets duplicated.
How can I stop him from doing that.

If you have a DontDestroyOnLoad on the FPC , you are preventing it from getting destroyed not only when you load the second level , but the first as well. So if the first level already has an FPC , the other FPC that is set not to destroy itself will be present.
You could save the FPC as a prefab and instantiate based on checking if an FPC already exists. Example:

var prefab:GameObject;
var spawn:Transform;
var fpcExists:boolean=false;
function Awake(){
for(var gameObj : GameObject in GameObject.FindObjectsOfType(GameObject))
    if(gameObj.name == "FirstPersonController"){
Instantiate (prefab, spawn.position, spawn.rotation);