This issue appears to be resolved after updating with the necessary follow up/cleanup steps: https://access.redhat.com/solutions/5118061
Removal of the NBDB/SBDB + deletion/restarting of all OVN master pods + removal/restart of all OVN worker pods in the openshift-ovn-kubernetes namespace has allowed the cluster to return to READY status + validation that no remaining duplicated/stale egressIP UUID's exist in the cluster after patch update to 4.8.29 (which includes fix for above).
We are confirming that this works on a secondary impacted cluster today, but has resolved primary issue and if confirmed we can mark this bug closed.
I will update this case with a confirmation shortly.
Although the wiping of dbs may have worked around the issue, there is still a bug here with ovnk. We should never have ended up with these duplicate rows in the first place. Flavio is working on a fix.