diff --git a/django/utils/datastructures.py b/django/utils/datastructures.py
index 4656f60f2b..30ce28d38d 100644
--- a/django/utils/datastructures.py
+++ b/django/utils/datastructures.py
@@ -120,9 +120,11 @@ class SortedDict(dict):
             self.keyOrder = data.keys()
         else:
             self.keyOrder = []
+            seen = set()
             for key, value in data:
-                if key not in self.keyOrder:
+                if key not in seen:
                     self.keyOrder.append(key)
+                    seen.add(key)
 
     def __deepcopy__(self, memo):
         return self.__class__([(key, deepcopy(value, memo))