diff --git a/france/france_rsvp/codes.py b/france/france_rsvp/codes.py
index 16f93ca..5fbe3e4 100644
--- a/france/france_rsvp/codes.py
+++ b/france/france_rsvp/codes.py
@@ -40,6 +40,11 @@ def _load_codes():
     errored_codes = {}
     for code in correct_codes:
         for error in list_errors(code):
+            if error in errored_codes:
+                raise ValueError("%s too close to %s" % (
+                    code,
+                    errored_codes[error],
+                ))
             errored_codes[error] = code
     assert (
         len(errored_codes)