The AAMC web site. With the Verbal Reasoning section inching ahead of the Physical Sciences, and the Biological Sciences. On a scale of 1-15, with 15 being the most important subtest, I'd give Verbal Reasoning a 15, Physical and Biological Sciences a 14, and the Writing Sample a 4. Writing Sample scores were assigned moderate weight during the admission deliberations and were considered at varied points in the selection process. No, I have never heard of such a thing being done.
Some schools use all sets of scores equally and note improvements.
Others consider only the most recent set of scores. Still others take an average of all sets of scores. Finally some schools use only the highest set of scores or the highest individual section scores. Of all methods, the first is the most common. You may wish to contact an institution's admission office directly for information about its procedures.