diff --git a/reclass_tools/helpers.py b/reclass_tools/helpers.py
index 5c3ea92..abbfb47 100644
--- a/reclass_tools/helpers.py
+++ b/reclass_tools/helpers.py
@@ -189,7 +189,7 @@
 
     # Add two lists
     if isinstance(obj_1, list) and isinstance(obj_2, list):
-        return obj_1 + obj_2
+        return obj_1 + list(set(obj_2) - set(obj_1))
 
     # Overwrite a value with new one
     return obj_2