This is also happening on one of our clients computers. It just started last week and the duplicates are only showing up on Macs and not Windows computers which tells me there is a bug in the Dropbox desktop app for Macs. Last week a DB tech gave the instructions to purge the Mac DB cache. It worked for one folder, but now the duplicates have returned. Many of the duplicates have the file name of (deleted .......) at the end.
This is a very serious problem especially for people with limited hard drive space, and seeing how long this has been going on tells me DB has no way to solve this. This is a bug in the Mac desktop app and has been going on for over a year looking at the date of the original post!!!!
If the next level of tech support cannot fix this tomorrow I will suggest to my client that they move to a different file sharing system and I will also see if a class action law suit can be started.