diff --git a/wlauto/core/extension.py b/wlauto/core/extension.py
index 3aed90dc..348a5b9f 100644
--- a/wlauto/core/extension.py
+++ b/wlauto/core/extension.py
@@ -82,7 +82,12 @@ class AttributeCollection(object):
         return p
 
     def __iadd__(self, other):
+        other = [self._to_attrcls(p) for p in other]
+        names = []
         for p in other:
+            if p.name in names:
+                raise ValueError("Duplicate '{}' {}".format(p.name, p.__class__.__name__.split('.')[-1]))
+            names.append(p.name)
             self.add(p)
         return self
 
@@ -687,4 +692,3 @@ class Module(Extension):
 
     def initialize(self, context):
         pass
-
diff --git a/wlauto/tests/test_extension.py b/wlauto/tests/test_extension.py
index 77e244f6..dee00cc4 100644
--- a/wlauto/tests/test_extension.py
+++ b/wlauto/tests/test_extension.py
@@ -320,6 +320,13 @@ class ParametersTest(TestCase):
         myext = _instantiate(MyOtherExtension, mandatory=1, optional='invalid')
         myext.validate()
 
+    @raises(ValueError)
+    def test_duplicate_param_override(self):
+        class DuplicateParamExtension(MyBaseExtension):  # pylint: disable=W0612
+            parameters = [
+                Parameter('food', override=True, default='cheese'),
+                Parameter('food', override=True, default='cheese'),
+            ]
 
 class ModuleTest(TestCase):
 
@@ -340,4 +347,3 @@ class ModuleTest(TestCase):
 def _instantiate(cls, *args, **kwargs):
     # Needed to get around Extension's __init__ checks
     return cls(*args, **kwargs)
-