Unity AssetPostProcessor to Identify Duplicate Assets of a Type

SaveableGUIDImporter.cs

using System.Collections.Generic;
using RedBlueGames.Tools.SaveGame;
using UnityEngine;
using UnityEditor;

/// <summary>
/// Assign new SavableGUIDs when they are imported
/// </summary>
public class SaveableGUIDImporter : AssetPostprocessor
{
    private static List<string> preProcessedAssets;

    private static List<string> PreProcessedAssets
    {
        get
        {
            if (preProcessedAssets == null)
            {
                preProcessedAssets = new List<string>();
            }

            return preProcessedAssets;
        }
    }

    void OnPreprocessAsset()
    {
        var asset = AssetDatabase.LoadAssetAtPath(assetImporter.assetPath, typeof(SaveableScriptableObject));
        if (asset == null)
        {
            PreProcessedAssets.Add(assetImporter.assetPath);
        }
    }

    static void OnPostprocessAllAssets(string[] importedAssets, string[] deletedAssets, string[] movedAssets, string[] movedFromAssetPaths)
    {
        foreach (string str in importedAssets)
        {
            if (PreProcessedAssets.Contains(str))
            {
                var asset = AssetDatabase.LoadAssetAtPath(str, typeof(SaveableScriptableObject));
                if (asset is SaveableScriptableObject)
                {
                    var assetAsSSO = (SaveableScriptableObject)asset;
                    var allSaveableSOs = LoadAllSaveableScriptableObjects();
                    foreach (var existingScriptableObject in allSaveableSOs)
                    {
                        if (existingScriptableObject.SaveableGUID == assetAsSSO.SaveableGUID &&
                            existingScriptableObject.name != asset.name)
                        {
                            assetAsSSO.EditorOnly_ResetGUID();
                            EditorUtility.SetDirty(asset);

                            Debug.Log($"Reset GUID on asset: {str} because it matched existing asset: {existingScriptableObject.name}");
                        }
                    }
                }

                PreProcessedAssets.Remove(str);
            }
        }

        PreProcessedAssets.Clear();
    }

    private static List<SaveableScriptableObject> LoadAllSaveableScriptableObjects()
    {
        var allSaveableSOs = new List<SaveableScriptableObject>();
        {
            var allSaveableSOGUIDs = UnityEditor.AssetDatabase.FindAssets("t: SaveableScriptableObject");
            foreach (var guid in allSaveableSOGUIDs)
            {
                var soPath = UnityEditor.AssetDatabase.GUIDToAssetPath(guid);
                var so = (SaveableScriptableObject)UnityEditor.AssetDatabase.LoadAssetAtPath(soPath, typeof(SaveableScriptableObject));
                allSaveableSOs.Add(so);
            }
        }

        return allSaveableSOs;
    }
}