aprea

-- delete any usermeta specific to the other subsites
delete from wp_usermeta where meta_key regexp '^wp_([0-9]+)_';

-- duplicate the wp_usermeta structure in a working data table,
-- but add a unique index for filtering out duplicates
create table _fix_usermeta like wp_usermeta;
alter table _fix_usermeta add unique(user_id, meta_key);

-- copy the site-specific usermeta, keeping only the last of each duplicate
insert into _fix_usermeta