Answer

PROBLEM

Error: Failed to update database. Error Message: A duplicate value cannot be inserted into a unique index

Locate the exact error in the mailessentials.controllists.xxx logs located in the \GFI\MailEssentials\Antispam\debuglogs folder

Example error:

"error ","MailEssentials.ControlLists.PWL","Error: Failed to update database. Error Message: A duplicate value cannot be inserted into a unique index. [ Table name = PWLBL_UserEmails,Constraint name = UQ__PWLBL_UserEmails__0000000000000059 ]"
"error ","MailEssentials.ControlLists.PWL","Error: Full Details: A duplicate value cannot be inserted into a unique index. [ Table name = PWLBL_UserEmails,Constraint name = UQ__PWLBL_UserEmails__0000000000000059 ]"

Run the following query:

SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS where table_name = 'PWLBL_UserEmails'

Run the following query to remove the corrupt entry

alter table [PWLBL_UserEmails] drop constraint UQ__PWLBL_UserEmails__00000000000000XX
The 00000000000000XX at the end of the query should be changed to match the number in the actual error, in this case it would be:
alter table [PWLBL_UserEmails] drop constraint UQ__PWLBL_UserEmails__0000000000000059
Remove the blank entry from the PWLBL_UserEmails table by right clicking it and selecting delete

For issues with tables other than the PWLBL_UserEmails table, use these commands.

SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS where table_name = 'PWLBL_Emails'
SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS where table_name = 'PWLBL_Users'
alter table [PWLBL_Emails] drop constraint UQ__PWLBL_Emails__00000000000000XX
alter table [PWLBL_Users] drop constraint UQ__PWLBL_Users__00000000000000XX

Blank or corrupt entry in a controllists.sdf table