There is several old discussion on this. But it seems there isn't a solution and those discussions are a while back. I started a new one here:
Because we are using dropbox, which will keep all revision history, we know what's the issue but don't know how to solve it.

From the screen shot, we know the black render elements are saved by DR render nodes(dropbox will indicate the conflict saving). The render elements saved by DR nodes overwrite the actual render elements from the main computer. Those elements which aren't black are what we recovered back by revision history from dropbox, which are saved by the main computer(the one really rendering)
Is there a way to stop the DR render node from saving render elements by the end of the rendering?