I just needed a new certificate for cvs and when I tried to import it into firefox, it failed. I noticed that both certificates have the serial number "00", which is afaik wrong. Maybe this prevented the import of the new certificate. It worked after I removed the old one.
When you do not track how many certificates an user has requested, afaik you can use a date string in the serial number, e.g. "2007-08-24T10:16" but I am not sure, whether non numeric character are allowed there.
This issue is tracked in the wiki pages for the new Fedora Account System, therefore I close this as duplicate.
to comment on this ticket.