diff --git a/Scripts/NodeDataCache.cs b/Scripts/NodeDataCache.cs
index 7061f17..f865ab2 100644
--- a/Scripts/NodeDataCache.cs
+++ b/Scripts/NodeDataCache.cs
@@ -199,7 +199,9 @@ namespace XNode {
                 if(formerlySerializedAsAttribute != null) {
                     if (formerlySerializedAsCache == null) formerlySerializedAsCache = new Dictionary<System.Type, Dictionary<string, string>>();
                     if (!formerlySerializedAsCache.ContainsKey(nodeType)) formerlySerializedAsCache.Add(nodeType, new Dictionary<string, string>());
-                    formerlySerializedAsCache[nodeType].Add(formerlySerializedAsAttribute.oldName, fieldInfo[i].Name);
+                    
+                    if (formerlySerializedAsCache[nodeType].ContainsKey(formerlySerializedAsAttribute.oldName)) Debug.LogError("Another FormerlySerializedAs with value '" + formerlySerializedAsAttribute.oldName + "' already exist on this node.");
+                    else formerlySerializedAsCache[nodeType].Add(formerlySerializedAsAttribute.oldName, fieldInfo[i].Name);
                 }
             }
         }