Getting this error when using collisions in javascript in 5.1 (Error: System.ArgumentException_

Hi all I am following this tutorial https://unity3d.com/learn/tutorials/projects/roll-a-ball/collecting-pick-up-objects

and am getting the following error:

Error Error: System.ArgumentException: An item with the same key has already been added. (Error: System.ArgumentException) (Assembly-UnityScript).

I looked it up and if I understood correctly it is a problem with unity 5.1 I tried double clicking to find the line and it is not found, I tried changing from debug to release and it gave me 3 more errors, I am a complete noob at javascript (learning on the fly) so I don’t really understand where I am going wrong. Thanks to anyone who can offer advice

#pragma strict

var rb: Rigidbody;
var speed : float;

function Start() {
	rb = GetComponent.<Rigidbody>();
}

function FixedUpdate () {
	var moveHorizontal : float = Input.GetAxis("Horizontal");
	var moveVertical : float = Input.GetAxis("Vertical");
	var movement : Vector3 = new Vector3 (moveHorizontal,0,moveVertical);
	rb.AddForce (movement * speed);
	function OnTriggerEnter (other : Collider) {
		if (other.gameObject.CompareTag("Pick Up")) {
			other.gameObject.SetActive (false);
		}
	}
}

Your script has an error stright out of the gates.

OnTriggerEnter function is inside of the FixedUpdate function, remove it so it looks like:

 #pragma strict
 
 var rb: Rigidbody;
 var speed : float;
 
 function Start() {
     rb = GetComponent.<Rigidbody>();
 }
 
 function FixedUpdate () {
     var moveHorizontal : float = Input.GetAxis("Horizontal");
     var moveVertical : float = Input.GetAxis("Vertical");
     var movement : Vector3 = new Vector3 (moveHorizontal,0,moveVertical);
     rb.AddForce (movement * speed);
 }

 function OnTriggerEnter (other : Collider) {
     if (other.gameObject.CompareTag("Pick Up")) {
         other.gameObject.SetActive (false);
     }
 }

Secondly if it continues after this, try closing monodevelop/visual studio and delete the .csproj files and .sln files in your folder, but only those files. Reopen the script in unity - subsequently your editor.