insert error

i am trying to insert these values
INSERT INTO assignments VALUES ('B0000','JE000', 'T2010', '12-NOV-12', '0');
INSERT INTO assignments VALUES ('C0000', 'EM002', 'T2010', '12-NOV-12', '0');
INSERT INTO assignments VALUES ('D0000','PG003', 'T2010', '12-NOV-12', '0');


but am getting this error

Error starting at line 11 in command:
INSERT INTO assignments VALUES ('D0000','PG003', 'T2010', '12-NOV-12', '0')
Error report:
SQL Error: ORA-00001: unique constraint (S12000606.SYS_C007001) violated
00001. 00000 - "unique constraint (%s.%s) violated"
*Cause: An UPDATE or INSERT statement attempted to insert a duplicate key.
For Trusted Oracle configured in DBMS MAC mode, you may see
this message if a duplicate entry exists at a different level.
*Action: Either remove the unique restriction or do not insert the key.




any one help?