Description of problem:
mockbuild with --no-clean by mock 0.6.12 immediately
stops with the following error.

Version-Release number of selected component (if applicable):
mock-0.6.12-1.fc7
yum-3.1.2-1.fc7

How reproducible:
100%

Steps to Reproduce:
1. rebuild some srpm by mockbuild with --no-clean option
2.
3.
  
Actual results:
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/state
DEBUG: ensuring dir /var/lib/mock/
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/var/lib/rpm
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/var/log
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/var/lock/rpm
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/dev
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/etc/rpm
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/tmp
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/var/tmp
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/etc/yum.repos.d
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/proc
DEBUG: mounting proc in /var/lib/mock/fedora-development-i386-core/root/proc
DEBUG: Executing /usr/sbin/mock-helper mount -t proc proc
/var/lib/mock/fedora-development-i386-core/root/proc
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/dev/pts
DEBUG: mounting devpts in /var/lib/mock/fedora-development-i386-core/root/dev/pts
DEBUG: Executing /usr/sbin/mock-helper mount -t devpts devpts
/var/lib/mock/fedora-development-i386-core/root/dev/pts
DEBUG: Executing /usr/sbin/mock-helper chroot
/var/lib/mock/fedora-development-i386-core/root /sbin/runuser - root -c "chown
500.500 /etc/yum.conf"
DEBUG: ensuring dir /var/lib/mock/fedora-development-i386-core/root/etc/yum
Traceback (most recent call last):
  File "/usr/bin/mock", line 1046, in <module>
    main()
  File "/usr/bin/mock", line 1043, in main
    do_rebuild(config_opts, srpms)
  File "/usr/bin/mock", line 910, in do_rebuild
    my.prep()
  File "/usr/bin/mock", line 254, in prep
    self._prep_install()
  File "/usr/bin/mock", line 659, in _prep_install
    os.symlink('../yum.conf', os.path.join(yumdir, 'yum.conf'))
OSError: [Errno 17] File exists

Expected results:
mockbuild proceeds...

Additional info:
yum-3.1.3-1.fc7 is broken and currently I use downgraded
yum 3.1.2