Well, I could manage that by removing the ID field and setting the Properties - Unique Values to Yes.
Don't think I'd do that if you're talking about prevention. The best prevention would be a composite index (probably 3 fields, not including ID autonumber if that's what you have).
As for removal, try using the Find Duplicates query wizard to create a base query of dupes that you can delete from. If memory serves, you leave out the ID field in such a query.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.