Fix metadata merge for BoxSets (#12583)

MediaBrowser.Providers/BoxSets/BoxSetMetadataService.cs

@@ -54,7 +54,14 @@ namespace MediaBrowser.Providers.BoxSets
 
             if (mergeMetadataSettings)
             {
-                targetItem.LinkedChildren = sourceItem.LinkedChildren;
+                if (replaceData || targetItem.LinkedChildren.Length == 0)
+                {
+                    targetItem.LinkedChildren = sourceItem.LinkedChildren;
+                }
+                else
+                {
+                    targetItem.LinkedChildren = sourceItem.LinkedChildren.Concat(targetItem.LinkedChildren).Distinct().ToArray();
+                }
             }
         }