Let\'s say I have three tables A, B, and C. Each has two columns: a primary key and some other piece of data. They each have the same number of rows. If I JOIN
This might sound like a really basic "DUH" answer, but make sure that the column you're using to Lookup from on the merging file is actually full of unique values!
I noticed earlier today that PowerQuery won't throw you an error (like in PowerPivot) and will happily allow you to run a Many-Many merge. This will result in multiple rows being produced for each record that matches with a non-unique value.