Steps to reproduce:
1. Open one project (Let's call it Project1)
2. Open another one at the same time (Project2)
3. Close or leave open the second project (Project2)
4. Click Reimport All on the first project (Project1)
4. Editor will try to open Project2 and fail saying it's already open on another instance of Unity or if you had closed it will just open Project2

Expected: Reimport all re-opens the project you're working on, not the other instance
Actual: The other instance somehow gets reimported and opened instead

Reproduced on 2018.1.0a5, 2017.3.0b11, 2017.2.0p3, 2017.1.2p3, 5.6.4.p3

Note: doesn't reproduce if the project was previously opened with another version.