diff --git a/swap_guids.py b/swap_guids.py
index 66ebba9..fdc7005 100644
--- a/swap_guids.py
+++ b/swap_guids.py
@@ -10,6 +10,8 @@ def modify_feed(context, feed):
     pprint.pprint(context)
 
     for article in context['articles']:
+        if article.title in articles:
+            raise Exception(f"Duplicate article title found: {article.title}")
         articles[article.title] = article
 
     for item in feed.items: