After installing Unity, on every first instance when user tries to import any package by double-clicking on it, a new instance of Unity gets created.

Note: New instance of Unity only gets created, on first instance after installation.

Steps to reproduce:
1)Install Unity
2) New Project
3)Try to import any package by double-clicking on it.

Workaround:
1)Restart Unity
2)Instead of importing by double-clicking a package externally, import by Assets > Import Package > Custom Package.

Works fine with : 2018.1.a7, 2018.1.a5, 2017.3.0f3, 2017.3.0f1

Regression introduced in : 2018.1.0b1

Environment: Windows. Doesn't happen on Mac.

Frequency: Always