If you're saying that any entry that has the same value in, say, field1,
field2, field3 and field4 is a duplicate, and you want to know about it,
create a query that includes all of those fields. Add an additional field
that you know will always have a value in it (doesn't matter if it's a
duplicate of one of the fields already in the query). Change your query into
a Totals query (you can do this through the View menu when the query's open
in Design view). That should add a new row (Total) to the grid, set to Group
By under every field. Change that Group By to Count under that extra field
you added to the query, and put > 1 as a criteria under that field.