Duplicate signatures clobbered in .bkp files

Summary

In .bkp files, it is possible to declare a signature multiple times within a unit, and it appears that only the last signature of a given name is considered; all others are ignored. I'm not certain if this is actually a bug, or how this is supposed to behave. cc @ezyang

Steps to reproduce

Write the following into a file test.bkp:

unit test where
  -- These two declarations of @signature A@ should be mergeable
  signature A where
    f :: Int
  signature A where
    g :: String

  -- These two declarations of @signature B@ are certainly not mergeable
  signature B where
    f :: Int
  signature B where
    f :: String

Then run ghc --backpack test.bkp, and inspect the generated .hi files with ghc --show-iface test/A.hi and ghc --show-iface test/B.hi.

Observed behaviour: GHC succeeds and writes out hi files for both A and B. A.hi lists only g :: String among its exports, and B.hi lists only f :: String among its exports.

Expected behavior

I am not entirely sure how signature merging is supposed to work in this scenario, but I expected one of two things to happen here:

1. GHC would point out that A and B have both been declared multiple times, and no .hi files would be produced at all
2. GHC would produce an A.hi file which listed both f :: Int and g :: String as exports, but would fail with an error at B, saying that the signatures were not mergeable

Environment

• GHC version used: reproduced on both 8.10.4 and 9.0.1

Optional:

• Operating System: Ubuntu linux
• System Architecture: x86_64