You are not allowed to apply for multiple provisional driving license (pdl) on NTSA TIMS portal. The best thing to do in such case is to visit the nearest NTSA offices to have your application canceled.
You may also email NTSA through dlqueries@ntsa.go.ke to have your PDL cancelled but it will take time for the issued to be addressed via email.
If you are going to Huduma Center (NTSA Desk) within Nairobi, makes sure that you have booked appointment for NTSA inquires.