mysql - SQL update that seems to require GROUP BY -

i have update i'd make table, like

update mytable left join worker on first=worker_first_name && last=worker_last_name group concat(first, '%%', last) set taint = count(pkey) > 1; 

but of course table_reference in update not allow group bys. how can set column? it's checking if first/last name combination occurs @ once in other table.

update mytable inner join (   select first, last, count(pkey) > 1 result   mytable   left join worker on first=worker_first_name , last=worker_last_name   group first, last) other     on other.first = mytable.first    , other.last = mytable.last set taint = other.result;