Sync software usually takes a conservative approach to deleting or merging records, and leaves duplicates lying around rather than risking deletion of vital user data. This is a good thing. What's bad is that the tools for housekeeping -- merging and deleting duplicates -- are generally rubbish. (I have the same problem with my phone's address book: masses of duplicates due to sync processes that conserve stuff. And trying to get rid of them using the tools provided turns out to be a tedious pain in the neck, requiring multiple mouse-clicks or focus changes per record.)
Further down the list we get into identity management issues. Nobody seems to have a really good handle on how we manage identity across multiple machines, much less how we manage esoteric stuff like family relations for delegating access to photos or music purchases or whatever.