plone/plone.app.layout — pull request #21 by hvelarde (2013-07-26 20:19:48+00:00)
Title: Implement a canonical link relation viewlet to be displayed by IHtmlHeadLinks viewlet manager

this will prevent web indexers from indexing the same object more than once, improving also the way these indexers deal with images and files.

for more information, see:

https://tools.ietf.org/html/rfc6596
https://support.google.com/webmasters/answer/139394?hl=en