Previously, when there are duplicate rows, I would first create a make table query, then do a select query on that new table using a count for whatever field is being duplicated. I know it sounds dumb, at least I'm making things happen. But this is really great, so here goes:
First you open up your query, so say it's a select query. Right click on the empty space beside the tables that you've chosen, and choose 'Properties'.
Next you should see a small box with the word 'Query Properties' at the top. Go to 'Unique Values' and choose 'Yes'. And there you have it.