Closing questions that are basically duplicates is obviously good. But, though the questions and their content are linked in perpetuity, there is no process for merging the best parts of each. This can leave the best content stranded in the closed questions instead of being transferred to the open question.
An example that is recent can be seen in these questions:
This was asked recently https://skeptics.stackexchange.com/questions/13870/normal-coke-vs-diet-coke-health
This is still open as well Are diet sodas healthier than normal sodas?
But this is closed despite being highly up-voted and containing the best content of all of the questions: Is diet soda less healthy than regular soda?
So, do we need to create a formal mechanism to allow answer mergers to preserve the best content in the open answers? Or do we just need a better way to select the question version that remains open?