Three times should be the charm here!
Go back and tell them it would be nice for them to really fix this. Ask them this time to please replace the screen or have them exchange the system with a new unit. That should do it. Have them commit to it this time (in writing if you can on the repair slip and your copy).
To make sure they do replace the screen how about making a small scratch mark on the lid somewhere so you have a way to tell (take a picture of it). When you go back to pick it up you'll have a way to tell. If they didn't replace it tell them you know its the same one (don't give your secret away yet). See what they say.