Multiple pull request heads can point at the same commit

[jcansdale]

C:\Source\github.com\dotnet\roslyn [master ≡]> git pr --list
  #29728 features/nullable-common
  #29729 features/nullable-common
  #29730 features/nullable-common
  #29731 features/nullable-common
  #31356 features/nullable-common
  #31364 using-var

416f53042eedcbc5cd620c4cd02caaccb35dc8c3	refs/pull/29728/head
e768014c071209d2fb85e25ae750849586b146e0	refs/pull/29728/merge
416f53042eedcbc5cd620c4cd02caaccb35dc8c3	refs/pull/29729/head
9fb60aeb65e47fc3a87966d9d9ddfe0452a67c8d	refs/pull/29729/merge
416f53042eedcbc5cd620c4cd02caaccb35dc8c3	refs/pull/29730/head
6d144cf672f3cf8255947ce56e96b41e6a4402a1	refs/pull/29730/merge
416f53042eedcbc5cd620c4cd02caaccb35dc8c3	refs/pull/29731/head
e107607ec9213cbf55c386fd0addbd2781900164	refs/pull/29731/merge
416f53042eedcbc5cd620c4cd02caaccb35dc8c3	refs/pull/31356/head
5632ed722d0b1b3ee3d94982bb00f989331a6626	refs/pull/31356/merge

The first few all appear to have the same commits:
https://github.com/dotnet/roslyn/pull/29728/commits
https://github.com/dotnet/roslyn/pull/29729/commits
https://github.com/dotnet/roslyn/pull/29730/commits
https://github.com/dotnet/roslyn/pull/29731/commits

The the last one doesn't exist:
https://github.com/dotnet/roslyn/pull/31356/commits

What is going on here and what should we do? 😕