Asset Bundle dependencies already in build

Using the Unity 5 asset bundle system, I am bundling several scenes into their own asset bundles. They all share some of the same assets. These shared assets are also being used in some scenes that are already being included in the build.

When I build the scene asset bundles, they each include a copy of these shared assets. Is there any way to not include assets that are already being used in the build? Or do I need to make a bundle with the shared assets and load that first?