How do I remove duplicate records

Query:

Select table_c.id_number, table_c.name, table_s.site_name,table_co.Contract_name 
FROM table_c , table_s, table_m, table_o, table_a, table_con 
  WHERE 
   table_s.objid = table_c.sobjid
   AND table_m.cobjid (+) = table_c.objid 
   AND table_o.objid (+) = table_m.olobjid   
   AND table_a.objid (+) = table_o.aobjid 
   AND table_co.objid (+) = table_a.conobjid;

Here I have 6 tables. table_c and table_s have one 2 one relationship. It is possible that for 1 table_o record, we can have 2 table_c records or may be no record.Since I need to take table_co values in SELECT along with table_c and table_s tables, I used left out join on all tables table_c->table_m->table_o->table_a->table_co.

Now on running this query it gives me duplicate records. I have millions of records in table_c so if i use distinct or Union ALL to remove duplicate records, my query takes very long time and this is not acceptable solution.

Can I right this query in such a way that it gives me unique records without performance issue.

Please note, this query is part of a sql view is used by external systems to get data.

Thanks in advance.