Why do you need the additional definite article in this sentence? Seems like "the cups" is already pretty defined by having koppene there.
It's needed before a definite adjective.
Of course - tusen takk!
Why the extra "m" in tomme? Are there any rules to follow here, or do you just have to learn all the exceptions?
The non definate adjective would be "Ser du tomme koppen."?
Ser du tomme kopper? - Do you see empty cups?