DrawDefaultEditor causing field to be serialized multiple times?

 The same field name is serialized multiple times in the class or it's parent class. This is not supported: Base(MonoBehaviour) m_Name
 UnityEditor.Editor:DrawDefaultInspector()
 GridInspector:OnInspectorGUI() (at Assets/Editor/GridInspector.cs:12)
 UnityEditor.DockArea:OnGUI()

I don't get it. Here's an example grid class below:

 [Serializable]
 public class Grid : MonoBehaviour {
 
     [SerializeField]
     private int m_Width;
     [SerializeField]
     private int m_Length;
     [SerializeField]
     private float m_TileSize;
     [SerializeField]
     private string m_Name;
     
     [SerializeField]
     private int[,] m_GridTile;
     
     public Grid ()
     {
         m_Name = "";
     }
     
     void OnGUI ()
     {
         m_Width = EditorGUILayout.IntField("Width", m_Width);
         m_Length = EditorGUILayout.IntField("Length", m_Length);
         m_TileSize = EditorGUILayout.FloatField("Tile Size", m_TileSize);
         //m_Name = EditorGUILayout.TextField("Name", m_Name);
     }
 }

Here's my inspector code

 [CustomEditor(typeof(Grid))]
 public class GridInspector : Editor
 {
     [SerializeField]
     private Grid grid;
     
     public override void OnInspectorGUI()
     {
         DrawDefaultInspector();   
         
         grid = (Grid)target;
      
         if (GUILayout.Button("Regenerate"))
         {
             //grid.MakeGrid();
         }
     }
 }

I would understand if I'm trying to re-declare the serialization but I'm not.

FUN FACT

If I change m_Name to m_Namee this works perfectly... is m_Name a hidden variable somewhere in the base class?