All supported PAS releases 15.x and above.
There are two timesheets being shown for the same time period and belonging to the same user in the 'Review & Approve' tab of the New UX timesheets because one of them is a posted timesheet, while the other is an open adjusted timesheet.
The open adjusted timesheet needs to either be deleted, or it can be processed as a normal timesheet. This would involve the user submitting the timesheet, having it approved, and it then being posted. Once it's processed as desired, there will only be one posted timesheet.
If the timesheet is moved to another status, such as submitted, approved, or returned, and not posted, then the timesheet will continue to be displayed there until it is posted for the time period or deleted.