Bug 1116839

Description of problem:
1) start PXE installation and add the "dnf" option to boot command line
2) on Welcome spoke click on "Continue" and confirm the pre release warning
3) the main hub starts loading and then Anaconda crashes with an exception

Version-Release number of selected component:
anaconda-21.45-1

The following was filed automatically by anaconda:
anaconda 21.45-1 exception report
Traceback (most recent call first):
  File "/usr/lib/python2.7/site-packages/dnf/comps.py", line 416, in group_install
    group.ui_name)
  File "/usr/lib/python2.7/site-packages/dnf/base.py", line 1384, in group_install
    trans = solver.group_install(grp, pkg_types, exclude)
  File "/tmp/updates/pyanaconda/packaging/dnfpayload.py", line 319, in _select_group
    self._base.group_install(grp, types, exclude=exclude)
  File "/tmp/updates/pyanaconda/packaging/dnfpayload.py", line 227, in _apply_selections
    self._select_group(group.name, default=default, optional=optional)
  File "/tmp/updates/pyanaconda/packaging/dnfpayload.py", line 402, in checkSoftwareSelection
    self._apply_selections()
  File "/tmp/updates/pyanaconda/ui/gui/spokes/software.py", line 117, in checkSoftwareSelection
    self.payload.checkSoftwareSelection()
  File "/usr/lib64/python2.7/threading.py", line 766, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/tmp/updates/pyanaconda/threads.py", line 227, in run
    threading.Thread.run(self, *args, **kwargs)
CompsError: Group 'Core' is already installed.

Additional info:
cmdline:        /usr/bin/python  /sbin/anaconda
cmdline_file:   initrd=/test/mk/fedora/initrd.img root=live:http://cobra02/trees/mk/my_comps_squashfs.img nokill updates=http://cobra02/ks/mk/master_updates.img resolution=1024x768 lang=en dnf BOOT_IMAGE=/test/mk/fedora/vmlinuz 
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         3.16.0-0.rc3.git2.2.fc21.x86_64
product:        Fedora
release:        Cannot get release name.
type:           anaconda
version:        21