Review the LDAP Mapping used in this instance. Review the option "Username exists in TPAM".
If this is configured to be "Report as Error" when a username already exists within TPAM, the warning "Skipping userID example_userid because there is an existing userid in TPAM with a different DN mapping" will be returned.
Review the options used on the Collision Strategy for this LDAP Mapping if different behavior is desired.
Full detail can be found in the TPAM Administrator Guide in the chapter "Auto Discovery - LDAP Integration"