Yes, you're missing something HUGE: you need more layers!
You need the badckground layer, and then above it the cup layer, with just the cup and its mask. Then you can duplicate the cup layer all you want.
This is really, Really, REALLY basic stuff. I recommend spending time learning the basics or you won't get anywhere in AE. It's too complicated to learn it intuitively by playing with it, or by saying to yourself, "I guess that's how it's supposed to work."
That's a surefire recipe for disaster.
There are PLENTY of materials out there to learn. Here's a great series right here on the COW:
http://blogs.adobe.com/aftereffects/2012/02/after-effects-basics-tutorials-...
When you go to the web address above, you will note that it's on the Adobe web site, written by Todd Kopriva. Todd happens to be Abobe's Man In Charge Of After Effects... Mr. After Effects himself. If Todd says those tutorials are good, you can take it to the bank.
Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA