Recently (the last few months or a year or so) I have noticed that Dropbox no longer seems to de-duplicate files across all users. Files I'm uploading which I know have been uploaded by another user to Dropbox recently, or are popular enough that it's very likely that at least one more person would have uploaded them, are uploaded fully from my end too. Has Dropbox issued any official statement about this?
For several years Dropbox has had global deduplication disabled. It seems to have been prompted by a software project "Dropship" that used global deduplication to trick Dropbox into giving people a file they didn't already have. While it could be used for legitimate purposes, Dropbox did not like it (one thing to think about is it could be used as an alternative to torrents for distributing pirated content using Dropbox's infrastructure).