Salary Slip Naming Series

I have already some salary slips in salary slip list which are from data import now when i make a payroll entry it gives me a error of duplicate entry the naming series for document is generating again from SAL/RD00296/00001, while last salary slip for this employee have a number SAL/RD00296/00024 RD00296 is the employee number.
A HELP WILL BE APRECIATED.