pythongh-131146: Fall back to month_name if standalone_month_names aren't distinct (pythonGH-137674)

Some systems reportedly don't expand '%OB' and '%Ob'.
In this case (and similar theoretically possible ones, like expanding to empty
string or 'OB'), fall back to the month_name & month_abbr.

Author: encukou

Lib/calendar.py

@@ -149,6 +149,14 @@ def __len__(self):
 except ValueError:
     standalone_month_name = month_name
     standalone_month_abbr = month_abbr
+else:
+    # Some systems that do not support '%OB' will keep it as-is (i.e.,
+    # we get [..., '%OB', '%OB', '%OB']), so for non-distinct names,
+    # we fall back to month_name/month_abbr.
+    if len(set(standalone_month_name)) != len(set(month_name)):
+        standalone_month_name = month_name
+    if len(set(standalone_month_abbr)) != len(set(month_abbr)):
+        standalone_month_abbr = month_abbr
 
 
 def isleap(year):