Problem for creating multiple group files

[jupiter]

Version: createrepo 0.9.8, yum 3.2.29.

Issue: createrepo -g group_file can only create one group file

Each time running "createrepo -g group.xml repo_directory" removes
previous group file metadata.

For example, "createrepo -g group1.xml repo_directory", yum grouplist
can see group1 content. Then if running second command "createrepo -g
group2.xml repo_directory", yum grouplist can only see group2, no
group1 any more.