Steps to reproduce:
1. Open attached project.
2. Enter the Play mode.
3. Press and hold left "Shift" key and notice that the "Console" window outputs this message: "GetKeyDown left shift", which indicated that the left "Shift" key was pressed.
4. While holding the left "Shift" key, press and hold right "Shift" key and notice that the "Console" window outputs this message: "GetKeyDown right shift".
5. Release the right "Shift" key.

Actual result:
The "Console" window did not output this message: "GetKeyUp right shift". Thus, the keyboard event was not registered.

Expected result:
The keyboard event should be registered when still holding left "Shift" key.

6. Release the left "Shift" key.
7. Notice that the keyboard events are registered (the "Console" window outputs: "GetKeyUp left shift" and "GetKeyUp right shift". However, keyboard events are being registered incorrectly, since the right "Shift" key was released before the left "Shift" key).

Reproduced with:
5.1.0f3, 5.2.4f1, 5.3.5f1, 5.3.5p4, 5.4.0b22.

Note1:
The issue is not reproducible when firstly releasing the key that was pressed first (in this case, it was the left "Shift" key).

Note2:
More information could be found here:
http://forum.unity3d.com/threads/unity-3-5-7f6-dual-shift-key-bug.178942/#post-1224229