Hadrian repeats type errors

Summary

Hadrian repeats type errors in its output. It appears that stderr is tee'd and then written out to stdout at the end.

This appears to be due to Shake and could be changed by changing how Hadrian calls Shake.

Steps to reproduce

I am using ./hadrian/build.sh under Alp's ghc.nix shell. To reproduce, introduce some small typo and build. Any error messages will show up twice.

Expected behavior

I would expect error messages to only be displayed once, to minimize output and avoid confusing programs like vim that parse error locations out of the build tool's output.

Workaround

If I just run ./hadrian/build.sh 2>/dev/null, I effectively only get the messages once.

Environment

GHC version used: HEAD (3bc6df32)

Optional:

Operating System: NixOS 19.03.172837.6c3826d1c93 (Koi)
System Architecture: x86-64

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information