SQL Joining tables to get other tables data

Please have a look at the following fiddle: http://sqlfiddle.com/#!9/ebddb/1/0 Here you see it's returning doubles (id, notchid and number should be unique). My Question: Why does it return doubles and how to prevent it? P.s. I found out that when there at least 6 records in notchtype, there are suddenly no duplicates anymore. And the null values should indeed be null. They have other values then an integer corresponding with notchtype.