Inner Join Returns Duplicate Entries?

I'm using a pretty simple inner join on two tables but the some of the same results are showing up twice. If I just remove the inner join all the results are uniquely present. For some reason the inner join creates about 10% more records that are all duplicate entries. Any idea what might cause this? :confused:
SELECT products.product_name
FROM products INNER JOIN categories ON products.category = categories.category_name
WHERE ((products.X)=True);