Portal Role Manager crashes with uninformative error message when duplicate users or groups exist

Portal Role Manager (/Portal/acl_users/portal_role_manager) crashes with an AssertionError when multiple users or groups exist with the same name. For example, if an LDAP groups exists called 'Administrators' and there is an internal Zope group called 'Administrators' the role manager will crash with the following exception:

Traceback (innermost last):
  Module ZPublisher.Publish, line 119, in publish
  Module ZPublisher.mapply, line 88, in mapply
  Module ZPublisher.Publish, line 42, in call_object
  Module Shared.DC.Scripts.Bindings, line 313, in __call__
  Module Shared.DC.Scripts.Bindings, line 350, in _bindAndExec
  Module Products.PageTemplates.PageTemplateFile, line 129, in _exec
  Module Products.Gloworm, line 28, in pt_render
  Module zope.pagetemplate.pagetemplate, line 117, in pt_render
  Module zope.tal.talinterpreter, line 271, in __call__
  Module zope.tal.talinterpreter, line 346, in interpret
  Module zope.tal.talinterpreter, line 536, in do_optTag_tal
  Module zope.tal.talinterpreter, line 521, in do_optTag
  Module zope.tal.talinterpreter, line 516, in no_tag
  Module zope.tal.talinterpreter, line 346, in interpret
  Module zope.tal.talinterpreter, line 855, in do_condition
  Module zope.tal.talinterpreter, line 346, in interpret
  Module zope.tal.talinterpreter, line 824, in do_loop_tal
  Module zope.tal.talinterpreter, line 346, in interpret
  Module zope.tal.talinterpreter, line 623, in do_insertText_tal
  Module Products.PageTemplates.Expressions, line 228, in evaluateText
  Module zope.tales.tales, line 696, in evaluate
   - URL: manage_roles
   - Line 53, Column 4
   - Expression: <PythonExpr ','.join( [x[1] for x in here.listAssignedPrincipals(info['id'])] )>
   - Names:
      {'container': <GroupAwareRoleManager at /intranet/acl_users/portal_role_manager>,
       'context': <GroupAwareRoleManager at /intranet/acl_users/portal_role_manager>,
       'default': <object object at 0xb7d33528>,
       'here': <GroupAwareRoleManager at /intranet/acl_users/portal_role_manager>,
       'loop': {'info': <Products.PageTemplates.Expressions.PathIterator object at 0xcee15ec>},
       'nothing': None,
       'options': {'args': ()},
       'repeat': <Products.PageTemplates.Expressions.SafeMapping object at 0xb92ee6c>,
       'request': <HTTPRequest, URL=http://ganymede-new:8080/intranet/acl_users/portal_role_manager/manage_roles>,
       'root': <Application at >,
       'template': <PageTemplateFile at /intranet/acl_users/portal_role_manager/manage_roles>,
       'user': <PropertiedUser 'siteadmin'>}
  Module Products.PageTemplates.ZRPythonExpr, line 49, in __call__
   - __traceback_info__: ','.join( [x[1] for x in here.listAssignedPrincipals(info['id'])] )
  Module PythonExpr, line 1, in <expression>
  Module Products.PluggableAuthService.plugins.ZODBRoleManager, line 282, in listAssignedPrincipals
AssertionError

The following error is also logged to the event log:

2009-03-26T11:52:51 ERROR PluggableAuthService searchPrincipals() returned more than one result for id=Administrators

I propose the attached patch to make this error message more informative, and reduce the time required to diagnose and resolve this problem. After applying the patch, if duplicate users or groups exist and someone accesses portal role manager, the following exception is thrown:

...
    * Module PythonExpr, line 1, in <expression>
    * Module Products.PluggableAuthService.plugins.ZODBRoleManager, line 284, in listAssignedPrincipals

MultiplePrincipalError: Multiple principals exist with the name 'Administrators'. Remove one of the duplicate groups or users.

The following message is logged to the event log:

2009-03-26T12:13:14 ERROR PluggableAuthService Multiple principals exist with the name 'Administrators'. Remove one of the duplicate groups or users.