registrar: raise a programming error on duplicated registering

Previous, registering different object with the same name would silently
overwrite the first value with the second one. We now detect the situation and
raise an error. No extension in test or core had the issues.

mercurial/registrar.py

@@ -8,6 +8,7 b''
 from __future__ import absolute_import
 
 from . import (
+    error,
     pycompat,
     util,
 )
@@ -50,6 +51,10 b' class _funcregistrarbase(object):'
     def _doregister(self, func, decl, *args, **kwargs):
         name = self._getname(decl)
 
+        if name in self._table:
+            msg = 'duplicate registration for name: "%s"' % name
+            raise error.ProgrammingError(msg)
+
         if func.__doc__ and not util.safehasattr(func, '_origdoc'):
             doc = func.__doc__.strip()
             func._origdoc = doc