The Gerrit accounts (scheme username:) are associated with LDAP accounts (scheme gerrit:) via All-Users.git refs/meta/external-ids.
Upstream provides a script to verify duplicate accounts due to case sensitiveness in a given namespace. For Gerrit local accounts we had a single one (username:Kaldari) which I have deleted (T323097).
In T307334#8394490, I have found out with 199 duplicate LDAP accounts.
Upstream provides a script contrib/find-duplicate-usernames.sh gerrit to find the duplicates which takes a scheme as parameter and it reports 398 entries (199 x 2).