Any of these will identify values appearing more than once:
select col1,count(*) from table_name a where ... group by col1 having count(*) > 1;
select col1 from table_name a where ... and 1 < (select count(*) from table_name b where b.col1 = a.col1);
select col1 from table_name a where ... and exists (select 'x' from table_name b where b.col1 = a.col1 and b.rowid != a.rowid);