HashSetthat comes built into .NET Framework, or you could build your own data structure. If you have tons and tons of files already in Google Drive, then implementing a Bloom filter in front of your lookup may help speed things up -- but that is an optimization for later.
HashSetfirst. See if it is fast enough.
Init: while (files available in Google Drive) download file data from Google Drive. compute hash file data put hash in data structure OnFileUploadAttempt: compute hash of file to be uploaded if data structure does not contains hash upload file