I don´t see how you could realize the current functionality of storing only one playlist/song no matter how often it is referenced by other playlists.

Quite simple really, a file copy from the empeg to the empeg would create a reference to the file or folder (additional database entry) rather than a copy just like it does with emplode.

This seems simple from the users view, but detecting such a "local" (to the USB drive) copy is not easy. While local moves are usually handled by just moving/renaming the link to the beginning of the file, local copies are usually handled the same way that non-local copies are: The OS reads the originating file and writes the destination file. There is no way for the device to know wether a write access is for a copy or a new file, except for accepting the whole new file and comparing it to existing ones after it got closed. Then again, the device has no really convenient way to know even when a file gets closed, it might just as well only be a file being flushed.
To get to the point of this all: A USB hard drive does not see any file opens, file writes or anything similar. It only sees sector/block reads/writes.

cu,
sven
(MkII 12GB blue now green, #080000113)