Why am I seeing notification for "Login must be unique"?
Oracle Service Cloud, All Releases
This notification indicates that the contacts table already contains a record with the login you are attempting to use here. The login field must be unique, thus, no duplicates are allowed.
In order to determine which contact already has that login value, you can use the Contact Search report (ID 8014). Search for a related contact record in question (perhaps there is another one already with this same email address) and view the details of that record in order to see if this person has more than one contact account and now trying to set match up the login values, which is not allowed. Keep in mind it is possible to have two records with the same email address, since that field is not unique.
If you want a report which allows you to search on "login", you can create a custom report as directed in this answer link:
That will allow you to search by any field you would like, by creating a filter on your report.