select column_name, count(column_name)
from table
group by column_name
having count (column_name) > 1;

This can be adjusted for selecting multiple columns as long as those are also in the group by, as in:

select column_one, column_two, count(*) from tablename group by column_one, column_two having count(column_one) > 1

Or even

...having count(*) > 1