transmogrify fails to create some group_members rows because of primary key id collision

The processing of the group_members table includes row-by-row callouts to the preRow function (check_group_membership), and that function has the side effect of trying to add new rows to the table (for memberships in owner groups). So there are inserts of rows with existing id values and creation of new rows without id values interleaved, and that right now is breaking.