Internal MSBuild Error: Already registered for build-time

Question

Tried to upgrade Visual Studio to 2015 Update 1 and it failed. Then started getting this error during my project build:

37>Unhandled Exception:MSB0001: Internal MSBuild Error: Already registered for build-time.
37>Error: MSB0001: Internal MSBuild Error: Already registered for build-time.

I thought it might be related to the Update 1, so I completely reinstalled Visual Studio. Nothing changed. I am able to build my solution (which has a mix of C# and F# projects) if I build the problem F# projects one at a time. But doing a rebuild on the entire solution gives me this error. Any idea where I can begin looking for answers?
thanks

Answer 1

So I got this problem when upgrading VS 2017 to version 15.8.0, and googled my way to a workaround. Although initially reported for VS 2015, and probably caused by another bug at that time, I have a feeling several developers will land here in the next few days.

A workaround is to set concurrent builds to 1. This can be done in VS 2017 in Tools/Options/Projects and Solutions/Build and Run, and then you see it at the top in the settings pane.

This obviously will slow down many builds, but for some of us it's probably easier (or perhaps needed) to use this workaround rather than downgrade while we wait for a fix, which I guess won't be long.