Function duplicate_commits_onto_parents 

pub fn duplicate_commits_onto_parents(
    mut_repo: &mut MutableRepo,
    target_commits: &[CommitId],
    target_descriptions: &HashMap<CommitId, String>,
) -> BackendResult<DuplicateCommitsStats>

Duplicates the given target_commits onto their original parents or other duplicated commits.

Commits in target_commits should be in reverse topological order (children before parents).

If target_descriptions is not empty, it will be consulted to retrieve the new descriptions of the target commits, falling back to the original if the map does not contain an entry for a given commit.