Inform Direct gives each new share certificate a unique number. 

It is not possible to use the same certificate number for two current share certificates. 

If you have used the incorrect number for a share certificate it can be edited or deleted before you create the new certificate.

To edit the certificate number please see How Do I Edit a Share Certificate?

To permanently delete a certificate, allowing the number to be used for a new record see How Do I Delete or Cancel a Share Certificate?