diff --git a/Core/Helpers/ItemHelper.cs b/Core/Helpers/ItemHelper.cs
index bddc0966..8125ff5f 100644
--- a/Core/Helpers/ItemHelper.cs
+++ b/Core/Helpers/ItemHelper.cs
@@ -528,7 +528,7 @@ public class ItemHelper
         // Second scan - Map parents to items.
         // Third scan - Resolve IDs.
         foreach (var item in items)
-            dupes[item.Id] = (dupes[item.Id] ?? 0) + 1;
+            dupes.Add(item.Id, (dupes.ContainsKey(item.Id) ? dupes[item.Id] : 0) + 1);
 
         foreach (var item in items)
         {