You could use a select statement like the following, to find out what employees exist more than once:

select emp_id, count(*)
from employee
group by emp_id
having count(*) > 1;

If you want to count the duplicate employees, you could do something like:

select count(*) from (
select emp_id, count(*)
from employee
group by emp_id
having count(*) > 1);

Here I’m assuming that an emp_id is the field used to identify an employee, but you can replace it but other field(s) as needed.