When a user’s username is changed in Salesforce, it produces a duplicate record in Gainsight’s User table because the User Connector job doesn’t let you select the SFDC User ID as a Key field - the only immutable field on the SFDC User record.
As a result, this is causing duplicate records to be displayed, exported when using SFDC User ID as a lookup value on dashboards, queries, etc.
Even though, yes, a SFDC Username must be unique - it can be changed on the same SFDC record. The SFDC User ID cannot. It should be the Key value.