Symptoms
During POA 5.5 pre-upgrade check one of the failed items is:
Precheck failed when checking identities duplicates.
Duplicated identities:
<E-MAIL>
You should: Rename identities login according auth_logins table or remove not existing identities
Cause
Due to inconsistency in database, in table identities there are two items with the same login field:
plesk=> select * from identities where login like '%<E-MAIL>%';
identity_id | system_id | login
-------------+-----------+--------------------
37152 | | <E-MAIL>
123108 | | <E-MAIL>
(2 rows)
plesk=> select * from auth_logins where login like '%<E-MAIL>%';
identity_id | login
-------------+--------------------
123108 | <E-MAIL>
(1 row)
Resolution
Remove the extra item(#37152 in this case) from table identities, and re-run the pre-upgrade check