Hi Tom,
First of all remember that Certificates are optional so not everone can be forced to collect theirs.
Secondly, if a student re-enrols onto a course for a re-take then they will find that their existing record of achievement will enable them to collect their certificate without retaking the assessment.
However, some time back we commissioned Chardelle to develop a certificate option for us which a) allows certificates to be re-issued and b) uses the latest pass date on the certificate itself. We have since granted
permission for this to be included in Moodle version 2 development. Printing an expiry date should be possible by adding a calculation to the certificate.php itself, or simply adding custom text to say " This certiticate is valid for 12 months from the date shown".
I agree with you that a mechanism for alerting students to re-take an assessment is a nice idea but I suspect that this would involve a completely new module and to be honest, should be attached to an assessable activity such as a quiz rather than an optional certificate.
Hope this helps.
Lester