If you find your JOIN command returning duplicate values and you’ve eliminated the usual suspects, check to see if the table being joined has a primary key. I had a 4 table JOIN statement where everything was fine when I did the first 3 JOINs, but once I added the 4th table, the results started returning multiple duplicate values. It turned out that table was missing a primary key.

After adding a primary key to the table, the query began functioning as expected.