Augment SerializableDictionary to allow temporary duplicates in Editor

* Also fixes an issue with the "Init Controllers" type lookup within
  InteractionModeManager.InitializeControllers() to find XRBaseControllers
  instead of XRControllers, since the legacy MRTK controllers derive from
  XRBaseControllers, and the GUI button previously wasn't finding any
  controller to initialize in the Inspector.

org.mixedrealitytoolkit.core/Utilities/SerializableDictionary.cs

@@ -21,23 +21,78 @@ public class SerializableDictionary<TKey, TValue> : Dictionary<TKey, TValue>, IS
 
         void ISerializationCallbackReceiver.OnBeforeSerialize()
         {
+#if !UNITY_EDITOR
             entries.Clear();
 
             foreach (KeyValuePair<TKey, TValue> pair in this)
             {
                 entries.Add(new SerializableDictionaryEntry(pair.Key, pair.Value));
             }
+#else
+            // While in Editor, the serialized entries list is managed differently and is not necessarily a 1:1 representation of
+            // the dictionary.  This allows for temporary duplicate keys, something the dictionary cannot do, while modifications
+            // are being made in the Inspector since the default behavior is to duplicate the last entry when adding a new one.
+
+            // Override the first entry that has a matching key from the dictionary, otherwise add to entries.
+            foreach (KeyValuePair<TKey, TValue> pair in this)
+            {
+                if (TryFindSerializableIndex(pair.Key, out int index))
+                {
+                    entries[index] = new SerializableDictionaryEntry(pair.Key, pair.Value);
+                }
+                else
+                {
+                    entries.Add(new SerializableDictionaryEntry(pair.Key, pair.Value));
+                }
+            }
+#endif
         }
 
         void ISerializationCallbackReceiver.OnAfterDeserialize()
         {
-            this.Clear();
+            base.Clear();
 
             foreach (SerializableDictionaryEntry entry in entries)
             {
-                this.Add(entry.Key, entry.Value);
+                base.TryAdd(entry.Key, entry.Value);
+            }
+        }
+
+#if UNITY_EDITOR
+        public new void Clear()
+        {
+            entries.Clear();
+            base.Clear();
+        }
+
+        public new bool Remove(TKey key, out TValue value)
+        {
+            if (base.Remove(key, out value))
+            {
+                if (TryFindSerializableIndex(key, out int index))
+                {
+                    entries.RemoveAt(index);
+                }
+
+                return true;
             }
+
+            return false;
+        }
+
+        public new bool Remove(TKey key)
+        {
+            return Remove(key, out _);
+        }
+
+        private bool TryFindSerializableIndex(TKey key, out int index)
+        {
+            var keyComparer = EqualityComparer<TKey>.Default;
+
+            index = entries.FindIndex((entry) => keyComparer.Equals(entry.Key, key));
+            return index != -1;
         }
+#endif
 
         [Serializable]
         private struct SerializableDictionaryEntry

org.mixedrealitytoolkit.input/Editor/Inspectors/InteractionModeManagerEditor.cs

@@ -2,6 +2,7 @@
 // Licensed under the BSD 3-Clause
 
 using MixedReality.Toolkit.Editor;
+using System.Collections.Generic;
 using UnityEditor;
 using UnityEngine;
 
@@ -27,6 +28,17 @@ public override void OnInspectorGUI()
             InteractionModeManager interactionModeManager = (InteractionModeManager)target;
 
             // Raise lots of errors if the interaction mode manager is configured incorrectly
+            var duplicateControllerMappings = GetDuplicateControllerMappings();
+            if (duplicateControllerMappings.Count > 0)
+            {
+                var duplicatedNameString = interactionModeManager.CompileDuplicatedNames(duplicateControllerMappings);
+
+                InspectorUIUtility.DrawError($"Duplicate controller mapping keys detected in the interaction mode manager on {interactionModeManager.gameObject.name}. " +
+                                    $"Please check the following controller mappings: {duplicatedNameString}");
+
+                GUI.color = InspectorUIUtility.ErrorColor;
+            }
+
             var duplicatedNames = interactionModeManager.GetDuplicateInteractionModes();
             if (duplicatedNames.Count > 0)
             {
@@ -58,5 +70,41 @@ public override void OnInspectorGUI()
 
             serializedObject.ApplyModifiedProperties();
         }
+
+        private HashSet<string> GetDuplicateControllerMappings()
+        {
+            HashSet<string> duplicatedNames = new HashSet<string>();
+
+            SerializedProperty controllerMapping = serializedObject.FindProperty("controllerMapping");
+            SerializedProperty entries = controllerMapping?.FindPropertyRelative("entries");
+
+            if (entries != null && entries.arraySize > 0)
+            {
+                HashSet<int> seenInstanceIDs = new HashSet<int>();
+
+                for (int i = 0; i < entries.arraySize; ++i)
+                {
+                    SerializedProperty entry = entries.GetArrayElementAtIndex(i);
+                    SerializedProperty key = entry.FindPropertyRelative("key");
+
+                    int instanceID = key != null && key.objectReferenceValue != null ?
+                        key.objectReferenceValue.GetInstanceID() : 0;
+
+                    if (seenInstanceIDs.Contains(instanceID))
+                    {
+                        string duplicateName = key != null && key.objectReferenceValue != null ?
+                            key.objectReferenceValue.name : "None (Game Object)";
+
+                        duplicatedNames.Add(duplicateName);
+                    }
+                    else
+                    {
+                        seenInstanceIDs.Add(instanceID);
+                    }
+                }
+            }
+
+            return duplicatedNames;
+        }
     }
 }

org.mixedrealitytoolkit.input/InteractionModes/InteractionModeManager.cs

@@ -71,7 +71,7 @@ public static InteractionModeManager Instance
         public void InitializeControllers()
         {
             controllerMapping.Clear();
-            foreach (XRController xrController in FindObjectUtility.FindObjectsByType<XRController>())
+            foreach (XRBaseController xrController in FindObjectUtility.FindObjectsByType<XRBaseController>())
             {
                 if (!controllerMapping.ContainsKey(xrController.gameObject))
                 {