Let's take an egregious example of this:
Alice and Bob want to clone a Pokemon.
They set up a trade between their console devices. Alice's device sends a copy of the data to Bob's device, then Bob's device sends a copy of theirs to Alice. As each device begins to save the results, Alice shuts off her device preventing the save. Bob does not. Bob now has Alice's Pokemon and Alice still has a copy of it on her device.
Suppose we try to set up a dirty bit before beginning the transfer and revert changes on next power on if the transfer has not completed. That doesn't seem to work either. After both parties complete they send completion acknowledgements to one another, Alice simply has to shut her device off while Bob receives and processes the acknowledgement. Bob gets the clone, Alice's database state reverts to previous.
It just seems to me like it is incredibly difficult to ensure that two equal peers with no mediator can sync up on a single piece of data if both are at risk of communication cutoff strategically employed at the right moment. But is it impossible?