tModLoader Error0101

c:\Users\Documents\My Games\Terraria\ModLoader\Mod Sources\TutorialMOD\Projectiles\BlueGemPROJ.cs(11,18) : error CS0101: The namespace 'TutorialMOD.Projectiles' already contains a definition for 'BlueGem'

My code is:

using System;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Terraria;
using Terraria.ID;
using Terraria.ModLoader;

namespace TutorialMOD.Projectiles
{

public class BlueGem : ModProjectile
{
public override void SetDefaults()...

tModLoader Error0101