Reproduction steps:
1. Open unitypackage attached (BundlesError.unitypackage)
2. Rebuld bundles using menu "Test/Build Bundle"
3. Load bundle in editor using menu "Test/Load Bundle". Try few times to make sure that it works.
4. Reimport script or make small change in it (Scripts/BundleTest).
5. Press "Test/Load Bundle" again.

Expected result: Asset bundle will be loaded.
Actual result: Asset bundle will not be loaded. Errors in console:
"The AssetBundle '.......' can't be loaded because another AssetBundle with the same files is already loaded."
"NullReferenceException: Object reference not set to an instance of an object"

Notes: EditorUtility.UnloadUnusedAssetsImmediate() is used before loading.

Reproduced on: 5.4.0f3, 5.4.4p4, 5.6.0p3

----------------------------------------------------------------------------------------------------------------------------------------------

To avoid this problem we added 2 new functions AssetBundle.GetAllLoadedAssetBundles() and AssetBundle.UnloadAllAssetBundles().
Those functions will be available to 5.6 (~5.6.2f1), 2017.1, 2017.2 and up.