Just how unique are the ppids generated by Appian? At what point would Appian st

Since in your scenario you were manually storing the process id's in the DB there is a big possibility of a problem in your logic. Since the DB is outside Appian and you're inserting them using a smart service if there's an error in your logic, or someone restarted the node or the node wasn't configured properly there can be inserts that you were not expecting. For this particular scenarios is the responsibility of the developers of your team to debug further; in terms of Appian's architecture process IDs are not re-used.

Since in your scenario you were manually storing the process id's in the DB there is a big possibility of a problem in your logic. Since the DB is outside Appian and you're inserting them using a smart service if there's an error in your logic, or someone restarted the node or the node wasn't configured properly there can be inserts that you were not expecting. For this particular scenarios is the responsibility of the developers of your team to debug further; in terms of Appian's architecture process IDs are not re-used.