How to fix conflict between non local keystone users

Issue

• It is impossible to authenticate users from keystone LDAP backend after user_id_attribute was changed in backend's configuration.
• It is impossible to authenticate user from keystone LDAP backend after some attribute of user's entry in LDAP that is used by keystone as user id was changed.
• It is impossible to authenticate users from existing LDAP backend after LDAP IP address was changed.

The following error in keystone.log is generated (the single-string message was parsed):

2018-06-19 15:32:15.396 236988 WARNING keystone.common.wsgi
    [req-ebce2c77-8140-49bf-ba55-e9c60c5bebc1 - - - - -]
        Conflict occurred attempting to store nonlocal_user - Duplicate Entry