Hmmm, ok ill give it another go.

When I did it the column that should show duplicate or no only had no in it.

So I added my name at the bottom of each column (a and b) and low and behold duplicate appeared next to my name.

I then changed one of the names at the top of column A to match a random name in column B, the no didn't change to duplicate, I reran the query (using the drag cell option) and the column still said no.