Using the checkout back button doubles the shipping breaks the complete checkout

In a current project I'm using the latest versions (dev) of Commerce Shipping 2 and Commerce Flat Rate to do the shipping calculation on a per country base. I created all the steps as instructed by Randy Fay in his video and it works very well.
There is just on thing. Every time I pass the shipping checkout page and then go back to one of the earlier checkout pages I see the shipping costs double and the check out is completely broken with the following error message:

EntityMetadataWrapperException: Unknown data property commerce_product. in EntityStructureWrapper->getPropertyInfo() (line 339 of /Users/<user>/Sites/drupal/<project>/www/sites/all/modules/contrib/entity/includes/entity.wrapper.inc).