How do I avoid duplicates when syncing computers?

Episode 1660 (53:20)

Lon from Apple Valley, CA
Computer Syncing

Lon wants to try and get rid of duplicate files that were created when he file syncs. Leo says that there are DeDupes out there, but Leo recommends 2nd Copy to file sync and merge duplicate copies. Another file sync is Free File Sync. Microsoft has RoboCopy, which is a command-line function.