diff --git a/masque/utils/autoslots.py b/masque/utils/autoslots.py
index e82d3db..cef8006 100644
--- a/masque/utils/autoslots.py
+++ b/masque/utils/autoslots.py
@@ -17,11 +17,12 @@ class AutoSlots(ABCMeta):
         for base in bases:
             parents |= set(base.mro())
 
-        slots = tuple(dctn.get('__slots__', ()))
+        slots = list(dctn.get('__slots__', ()))
         for parent in parents:
             if not hasattr(parent, '__annotations__'):
                 continue
-            slots += tuple(parent.__annotations__.keys())
+            slots.extend(parent.__annotations__.keys())
 
-        dctn['__slots__'] = slots
+        # Deduplicate (dict to preserve order)
+        dctn['__slots__'] = tuple(dict.fromkeys(slots))
         return super().__new__(cls, name, bases, dctn)