I observed it from my own usage, and I'm hard pressed to see any alternative.

I don't see how it shouldn't be good practice. The only information you're potentially giving away is that someone else that has a dropbox is in possession of the same file as you.

If I were the RIAA or MPAA, this seems to leak enough information to confirm that my copyrighted material is stored on dropbox, which might be enough to get a subpeona issued.

If dropbox client were to still force the upload to happen, and only discard the object afterwards, this "duplicate file exists" information wouldn't be leaked. Presumably, they want to detect duplicates within an account and avoid the transfer in that case in the interest of user experience, but if I've told dropbox to upload avatar, I'm presumably OK with the transfer happening, even if it's not strictly needed because someone else has already uploaded it.