Yeah if that's true that'd be weird if it's between users. The concept makes sense from a conservatory standpoint but unless they're the exact same file that isn't private and won't get modified, this wouldn't make any sense ...
Yeah I can see it working that way for the public folder and shared folder. For example with some of my classmates we have a shared folder between us. Files stored in there count against all of our max storage space but I can promise you Dropbox only has one copy of each of those files saved.
Great tool when working on group projects.