Constrain duplicate patterns

Hi,

Is there a way to make postgresql report an error if when
inserting data, a multiple-column combination is duplicated?

CREATE TABLE parts_vendors (
     part_id integer references parts (part_id),
     vendor_id integer references vendors (vendor_id),
     ...
)

part_id  vendor_id
------------------
1        3
2        5 <--+
7        9    |
2        5 <--+-- duplicate pattern
...