The duplicate userid's may exist only when 1 is active and the other is inactive. Identify and rename the duplicated userid's. This can be done from:
1) Database level by running: select userid, count(*) from ca_contact group by userid having count(*) > 1
2) CA Service Desk Manager > Security and Role Management > Contacts Search for active and inactive users > Locate the duplicate userid's.
3) Once the duplicates are identified, rename the inactive userid's and this will solve the issue.