diff --git a/pelican/readers.py b/pelican/readers.py
index a7712f0b..74b617e6 100644
--- a/pelican/readers.py
+++ b/pelican/readers.py
@@ -40,7 +40,7 @@ DUPLICATES_DEFINITIONS_ALLOWED = {
     'category': False,
     'author': False,
     'save_as': False,
-    'URL': False,
+    'url': False,
     'authors': False,
     'slug': False
 }