Let me start by saying that there is no 100 percent optimal solution to the image issues in RWD. This is because of the current lack of support for the recommended properties, or because there's a double download of assets. Granted, Dave Newton's article in http://ww1.smashingmagzine.com/, How To Avoid Duplicate Downloads In Responsive Images, tries to address this issue (http://www.smashingmagazine.com/2013/05/10/how-to-avoid-duplicate-downloads-in-responsive-images/).
However, that solution is very verbose. If you have to work with many images, this solution may not be the best option and allowing a double download starts making more sense. Every project is different, so trying to make the most informed decisions possible is incredibly important.
As soon as browser vendors decide to fully support any of the solutions mentioned here, there won't be a need to worry about double downloads or polyfills of any kind.