Problem with Duplicate Keys

I have a table with following fields

FIELDKEY int autoincrement
FIELD1 varchar
FIELD2 varchar
FIELD3 varchar
FIELD4 varchar

When an insert is done twice with the values 1,2,3,4. The row for FIELD1...FIELD4 are populated with 1,2,3,4 twice. Is there a way to
make these values four fields unique such that it would not allow duplicates
for these four field combinations?

Thanks.