Bug: `git commit --fixup` with duplicate commit messages

Hi,

I believe I have found a bug in `git commit --fixup`.

Steps to reproduce:
1. Create a git history with two commits (A and B) with the same
commit message (e.g. foo)
2. Create a new commit using `git commit --fixup {SHA}`, referring to
the last commit SHA
3. Run an interactive rebase

Expected: the fixup commit should appear below the last commit in the todo list.

Actual: the fixup commit appears below the first commit in the todo list.

It seems that the fixup commit is moved below the most recent commit with a matching commit message, however in this case there are duplicate commit messages. I would expect the fixup commit to be moved below the commit SHA I specified when I ran `git commit --fixup`.

Thanks, Olly