You cannot install two certificates with the same serial number from the same certification authority.
To resolve this issue, you must install certificates with the unique serial number from the same certification authority. Each certification authority maintains a list of unique serial numbers for the certificates it has issued.
You might also receive this error if you have installed the same certificate twice and are now trying to update the certificate. To resolve this issue, restart the NetScaler, unbind the certificate from the resources that it is bound to, remove the certificate and then update and bind the certificate.