AssetPostprocessor.OnPostprocessAllAssets fires two 'import' notifications for every dirty asset on 'Save Project'.

Reproduce:
1) Open attached sample project
2) 'Editor/AssetFileWatcher.cs' is the implementation of the custom AssetPostprocessor, which basically only Debug.Log's what has been processed.
3) Click 'File > Save Project'. Since no assets have been modifed yet, nothing is written to the Console.
4) Change 'Assets/New Material', so it gets marked as dirty in the editor. I changed the x-tiling value of the material.
5) Click 'File > Save Project'.

Observe that the AssetFileWatcher.OnPostprocessAllAssets() method has been called two times for the same asset, you should see two logs in the Console window that did output the same text.

Actually I expected to get only the 'imported' notification once for an asset, not multiple times. I guess this as a bug.