SQL & PL/SQL

Table has many records, some records are also duplicate (vm_no,vm_date)
now I am adding a constraint

SQL> alter table vouchermaster
  2  add constraint un_vm_vd unique (vm_no,vm_date);
add constraint un_vm_vd unique (vm_no,vm_date)
               *
ERROR at line 2:
ORA-02299: cannot validate (UN_VM_VD) - duplicate keys found

First time I am using UN_VM_VD constraint name

when I check my table duplicate records

SQL> select vm_no,vm_date from vouchermaster
  2  group by vm_no,vm_date
  3  having count(1) >1;

     VM_NO VM_DATE
---------- ---------
         5 18-MAY-10
         7 19-MAY-10
        18 19-MAY-10
        38 24-MAY-10

guide me can I add constraint Unique or primary key without data lose with force.