I think it's impossible to have deduplication (a system to prevent duplication) in encrypted data with different key for each user (the same file from different

I think it's impossible to have deduplication (a system to prevent duplication) in encrypted data with different key for each user (the same file from different user will be encrypted differently) without dropbox storing the key (to decrypt back). Which means they have access to the files. We should know this from the very beginning. If it wasn't, how to check the files for deduplication? Enlighten me please.