I entered the details of four staff twice. ( became eight staff )
How to rectify?
Just to clarify, do you mean you've created 8 employees in the STP app or have you entered the pay event details of the 4 staff in twice and sent it in a submission?
I created 8 employees in the STP
Go into the duplicated employee's profile ie. the one that isn't required, and tap on the menu button in the top right corner and select 'Delete <employee name>'
Tap on the Confirmation and the employee will be deleted from your app account.