How to undo "git commit --amend" done instead of "git commit"

I accidentally amended my previous commit. The commit should have been separate to keep history of the changes I made to a particular file.

Is there a way to undo that last commit? If I do something like git reset --hard HEAD^, the first commit also is undone.

(I have not yet pushed to any remote directories)

What you need to do is to create a new commit with the same details as the current HEAD commit, but with the parent as the previous version of HEAD. git reset --soft will move the branch pointer so that the next commit happens on top of a different commit from where the current branch head is now.

# Move the current head so that it's pointing at the old commit
# Leave the index intact for redoing the commit.
# HEAD@{1} gives you "the commit that HEAD pointed at before 
# it was moved to where it currently points at". Note that this is
# different from HEAD~1, which gives you "the commit that is the
# parent node of the commit that HEAD is currently pointing to."
git reset --soft HEAD@{1}

# commit the current tree using the commit details of the previous
# HEAD commit. (Note that HEAD@{1} is pointing somewhere different from the
# previous command. It's now pointing at the erroneously amended commit.)
git commit -C HEAD@{1}

use the ref-log:

git branch fixing-things HEAD@{1}
git reset fixing-things

you should then have all your previously amended changes only in your working copy and can commit again

to see a full list of previous indices type git reflog

Find your amended commits by:

git log --reflog

^{Note: You may add --patch to see the body of the commits for clarity. Same as git reflog.}

then reset your HEAD to any previous commit at the point it was fine by:

git reset SHA1 --hard

^{Note: Replace SHA1 with your real commit hash. Also note that this command will lose any uncommitted changes, so you may stash them before. Alternatively, use --soft instead to retain the latest changes and then commit them.}

Then cherry-pick the other commit that you need on top of it:

git cherry-pick SHA1

None of these answers with the use of HEAD@{1} worked out for me, so here's my solution:

git reflog

d0c9f22 HEAD@{0}: commit (amend): [Feature] - ABC Commit Description 
c296452 HEAD@{1}: commit: [Feature] - ABC Commit Description 

git reset --soft c296452

Your staging environment will now contain all of the changes that you accidentally merged with the c296452 commit.