To reproduce:
1. Download user's attached Unity package
2. Open any Unity 2018.1 beta version
3. Create a new or open an existing project
4. Right after it has finished loading double-click the package file

Expected result: Unity opens an import window containing the files in the package
Actual result: A new instance of most recently installed Unity is opened

Notes:
- Tested on 8, reproduced with 4 machines;
- The best chance to reproduce this is to install any 2017.2 version, open any 2018.1 beta and then double-click the package file;
- Only reproduces on Windows;
- Not interacting with the Editor window helps to reproduce.