It's not only the chainmail. If memory serves there are other items that are both in the Stash and the Emporium (others of the armor, the Scimitar, etc.).
You'll note that you do not have sufficient armor to outfit a full party of 5 in the Emporium. The ability to purchase the same armor that you have in your Stash from the Emporium allows you to outfit more heroes in better armor.
You're absolutly right. I dind't realize there are other 3 items in both places: scimitar, broad sword ande stubbed leather.