Prevent duplicate data entry

I would like to make sure there are no duplicate data entries in my Oracle 9i table (called MainTable) which has an Id field that is the primary key, ValData with a varchar data type, Fid and Fid2 are number data types.

Id ValData Fid Fid2

1    abc       34    2
2    efg       23    34
3    zeo       25    43

Sometimes someone can enter a duplicate ValData, Fid and Fid2 and it will end up like this:

Id ValData Fid Fid2

1    abc       34    2
2    efg       23    34
3    zeo       25    43
4    zeo       25    43

What constraints or restrictions can I place on the MainTable where it will never allow a duplicate entry into the table? I would like to do this somehow in the database. If someone tries to enter a duplicate I should get a error message or something to indicate an attempt to enter duplicate data. Received on Sun Oct 21 2007 - 18:37:49 CDT