You can make a new table occurrence of tbCerts, name it tbCerts|SameType and link it like this:
tbCerts::SerialNo = tbCerts|SameType::SerialNo AND
tbCerts::CertType = tbCerts|SameType::CertType
You can specify a sort order for this relationship that sorts tbCerts|SameType in Descending order by CertDate
then, when your context is a record in tbCerts, a reference to tbCerts|SameType will refer to the most recent tbCerts record for the same item of the same type.
But be careful of setting a field such as CertRecent to mark the most recent record of this serialNo and type as you will need to clear this value for the previous record as well as set it for the most recent record. The above relationship, BTW, can be used with a portal filter to drop out the "old" certifications and keep the most recent ones without any additional data being recorded in the fields. You'd use this portal filter:
tbCerts::ID = tbCerts|SameType::ID
Worked perfect. The portal filter was a way more elegant method than the checkbox.