Insert non duplicate data

I'm trying to construct an insert statement which will only insert data if it
hasn't been inserted before in the same table, i.e. insert x,y into table
where not exists (select x,y from table), or some such statement. Is there an
easy way of doing this - maybe an if statement that selects x,y and if they
don't exist they are inserted? What would the syntax of such a query be?
.