Virtually anything can be copied. Of course, whether they can get the most out of the device is another thing.
Last year, midway through the season, McLaren tried to copy RBR's rear end (exhaust system) and they just couldn't get a grip on this new device.
Other people tried to copy McLaren's F-Duct, but no team were able to extract as much from the F-Duct as McLaren.
I think its a lot easier to build a brand new car (during the Winter) from the ground up, incorporating a "trick device". When you are retro-fitting an existing car with a new device, it becomes more difficult as compromises are made.
In any case, most teams like to keep their trick parts, a secret for as long as possible. The moment the trick part is revealed, other teams get to work on attempting to incorporate it into their existing car. Some teams decide against it, but others do successfully incorporate it. The double diffuser in 2009 is a fantastic example of this.