FOLIO is allowing more than one user to have the same externalSystemId. This field is missing a unique constraint. Please add one like the existing ones for barcode and username.
The Check-Out settings allow to use the external system id as a unique patron identifier: https://folio-snapshot.dev.folio.org/settings/circulation/checkout
If external system id is not unique the check-out will pick one of the multiple matches without any warning that it might be the wrong patron.
The upgrade will fail if existing patrons have non-unique external system id with an error message containing the duplicate value. This is mentioned in the https://wiki.folio.org/display/REL/Lotus+%28R1+2022%29+Release+Notes
There is no automated way to fix duplicates during migration therefore the failure is the only option we have.