When you first apply for the statement of examination results you need to have a job appointment because the statement is sent to the place of application (i.e where the certificates are required to confirm that you sat for the examinations in question e.g KCSE Exams results ).
NB Every time you apply for the job and requested to give out your certificates you are required to go to KNEC and get your statement of examination results (Fees required do apply).
My advice is if you have not lost your certificate just keep it safe.