Upgrade from 15.7.6 to 15.8
Open solution
Build (Rebuild, Clean/Rebuild, whatever)

1>------ Rebuild All started: Project: ****************, Configuration: Debug Any CPU ------
2>------ Rebuild All started: Project: ********************, Configuration: Debug Any CPU ------
2>Unhandled Exception:MSB0001: Internal MSBuild Error: Already registered for build-time.
2>Error: MSB0001: Internal MSBuild Error: Already registered for build-time..

I emptied all VS cache folders and ran a repair install, no luck.
I could repro with a dummy solution with just an empty console app and 2 empty libraries, all on .NET Framework (not .NET Core or Standard). However after closing and reopening VS on that solution, I could no longer repro.
ConsoleApplication2.zip

I can always repro on our solution with 90+ projects (mix F#/C#, all .NET Framework).

Tools->Options->Projects and Solutions->Build and Run->
Reducing the number of parallel projects to 1 was a workaround.

Another weird thing I noticed is that in the Project Properties->Application, the Target Framework dropdown is empty:

Since this version seems completely broken and I need to work, I'm going to downgrade to the previous release using these instructions https://docs.microsoft.com/en-us/visualstudio/productinfo/installing-an-earlier-release-of-vs2017

EDIT: The problem is gone after reverting back to 15.7.6.