DIGITAL SIGNATURE RELATED

Hi frnds,

I have a Query about DSC
One of my Client already have DSC to use PF purpose and also used by me for IT & GST
The DSC was kept in Client office, Whenever it is needed they send through courier or person, this is routine process.
My query is Whether we have to apply for another DSC and used for IT and GST purpose? Even though he already have a DSC?
If I apply a DSC its violation or invaild?
Kindly clarify
Its Urgent