Newbie here (and yes, I have watched the vast majority of training vids...) pls be gentle....
I have data of listeners who are streaming music.
My guess is that when you created the listeners set for Week17, the userID and the week number were included in the set--same thing for the set when created in Week16. Edit both of these sets and X out the Week Number columns. Then, create the combined set.
Hope this helps!