How to fix the Unity C# error 'Type 'PlayerMovement' already defines a member called > 'Update' with the same parameter types''

Question

The error messages say

'Assets/Scripts/PlayerMovement.cs(5,14): error CS0101: The namespace '<global namespace>' already contains a definition for 'PlayerMovement' Assets/Scripts/PlayerMovement.cs(12,10): error CS0111: Type 'PlayerMovement' already defines a member called 'Start' with the same parameter types Assets/Scripts/PlayerMovement.cs(18,10): error CS0111: Type 'PlayerMovement' already defines a member called 'Update' with the same parameter types'

Here's my code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerMovement : MonoBehaviour
{
    Rigidbody rb;

    [SerializeField] float movementSpeed = 6f;
    [SerializeField] float jumpForce = 5f;

    [SerializeField] Transform groundCheck;
    [SerializeField] LayerMask ground;

    // Start is called before the first frame update
    void Start() {  
        rb = GetComponent<Rigidbody>();
    }

    // Update is called once per frame
    void Update()
    {

        float horizontalInput = Input.GetAxis("Horizontal");
        float verticalInput = Input.GetAxis("Vertical");

        rb.velocity = new Vector3(horizontalInput * movementSpeed, rb.velocity.y, verticalInput * movementSpeed);

        if (Input.GetButtonDown("Jump")) {
            rb.velocity = new Vector3(rb.velocity.x, jumpForce, rb.velocity.z);
        }  
    }
 
    bool IsGrounded() {
        Physics.CheckSphere(groundCheck.position, .1f, ground);
    }
    
}

How can I fix this? This is the only script in my project. I've looked through all similar questions and nothing has helped. I'm a beginner.