How to reproduce:
1. Create new Unity project
2. Go to Edit --> Project Settings --> Player
3. Change "Scripting Runtime Version" setting from Stable(.NET 3.5 Equivavalent) to Experimental(.NET 4.6 Equivavalent)
4. Import any package (Standard Assets for example)

Expected result: Package Import has a folder tree view and doesn't show copies of folders in the package
Actual result: Package Import window does not show folder tree view and shows repetitions of all the folders

Note: "Scripting Runtime Version" setting was introduced in 2017.1.0a6

Reproducible with - 2017.1.0b5, 2017.1.0a6

Fixed in 2017.2.0a3