I am getting this error, why?

The error reads - FirstPersonController.cs(5,14): error CS0101: The namespace global::' already contains a definition for FirstPersonController'

I only got this error after adding the crouching section at the bottom.

Here is my code -

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof(CharacterController))]
 public class FirstPersonController {
     
     public float movementSpeed = 5.0f;
     public float mouseSensitivity = 5.0f;
     public float jumpSpeed = 20.0f;
     
     float verticalRotation = 0;
     public float upDownRange = 60.0f;
     
     float verticalVelocity = 0;
     
     CharacterController characterController;
     
     // Use this for initialization
     void Start () {
         Screen.lockCursor = true;
         characterController = GetComponent<CharacterController>();
     }
     
     // Update is called once per frame
     void Update () {
         // Rotation
         
         float rotLeftRight = Input.GetAxis("Mouse X") * mouseSensitivity;
         transform.Rotate(0, rotLeftRight, 0);
 
         
         verticalRotation -= Input.GetAxis("Mouse Y") * mouseSensitivity;
         verticalRotation = Mathf.Clamp(verticalRotation, -upDownRange, upDownRange);
         Camera.main.transform.localRotation = Quaternion.Euler(verticalRotation, 0, 0);
         
 
         // Movement
         
         float forwardSpeed = Input.GetAxis("Vertical") * movementSpeed;
         float sideSpeed = Input.GetAxis("Horizontal") * movementSpeed;
         
         verticalVelocity += Physics.gravity.y * Time.deltaTime;
         
         if( characterController.isGrounded && Input.GetButton("Jump") ) {
             verticalVelocity = jumpSpeed;
         }
         
         Vector3 speed = new Vector3( sideSpeed, verticalVelocity, forwardSpeed );
         
         speed = transform.rotation * speed;
         
         
         characterController.Move( speed * Time.deltaTime );
 
 
         // Crouching
 
         if( characterController.isGrounded && Input.GetButton("Left Shift") || Input.GetKey("right shift")){
 
             transform.Translate(Vector3.down * movementSpeed * Time.deltaTime);
     }
 
 }