Description of problem:
Boot anaconda with an existing mdraid RAID1. Crash.

Reproduced with iso Fedora-Server-netinst-x86_64-25-1.3.iso. It has anaconda-25.20.8-1.

Reproduce:

Start with a system with two equal sized empty disks, sda and sdb. Boot inst.rescue. Set up a RAID1 by something like this (this is from memory, not pasted from actual commands)

fdisk /dev/sda
n (new)
p (primary)
1 (partition 1)
<enter> (default start block)
+1024M (size)
t (type)
fd (linux raid autodetect)
n (new)
p (primary)
2 (partition 2)
<enter> (default next start block)
<enter> (size: rest of disk)
t (type)
fd (linux raid autodetect)

w (write and exit)

sfdisk -dump /dev/sda > dump
sfdisk /dev/sdb < dump

mdadm --create /dev/md/BOOT --level=1 --raid-disks=2 /dev/sda1 /dev/sdb1
mdadm --create /dev/md/SYS --level=1 --raid-disks=2 /dev/sda2 /dev/sdb2

Check that it syncs by

cat /proc/mdstat
cat /proc/mdstat
cat /proc/mdstat

Let it sync for some seconds, then reboot, and run the normal GUI installer. It crashes at startup.

Version-Release number of selected component:
anaconda-25.20.8-1

The following was filed automatically by anaconda:
anaconda 25.20.8-1 exception report
Traceback (most recent call first):
  File "/usr/lib/python3.5/site-packages/blivet/devicetree.py", line 148, in _add_device
    raise ValueError("device is already in tree")
  File "/usr/lib/python3.5/site-packages/blivet/threads.py", line 45, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.5/site-packages/blivet/populator/helpers/partition.py", line 111, in run
    self._devicetree._add_device(device)
  File "/usr/lib/python3.5/site-packages/blivet/populator/populator.py", line 306, in handle_device
    device = helper_class(self, info).run()
  File "/usr/lib/python3.5/site-packages/blivet/threads.py", line 45, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.5/site-packages/blivet/populator/populator.py", line 154, in _add_slave_devices
    self.handle_device(slave_info)
  File "/usr/lib/python3.5/site-packages/blivet/threads.py", line 45, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.5/site-packages/blivet/populator/helpers/mdraid.py", line 55, in run
    self._devicetree._add_slave_devices(self.data)
  File "/usr/lib/python3.5/site-packages/blivet/populator/populator.py", line 306, in handle_device
    device = helper_class(self, info).run()
  File "/usr/lib/python3.5/site-packages/blivet/threads.py", line 45, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.5/site-packages/blivet/populator/helpers/mdraid.py", line 216, in run
    self._devicetree.handle_device(array_info, update_orig_fmt=True)
  File "/usr/lib/python3.5/site-packages/blivet/populator/populator.py", line 345, in handle_format
    helper_class(self, info, device).run()
  File "/usr/lib/python3.5/site-packages/blivet/threads.py", line 45, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.5/site-packages/blivet/populator/populator.py", line 318, in handle_device
    self.handle_format(info, device)
  File "/usr/lib/python3.5/site-packages/blivet/threads.py", line 45, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.5/site-packages/blivet/populator/populator.py", line 518, in _populate
    self.handle_device(dev)
  File "/usr/lib/python3.5/site-packages/blivet/threads.py", line 45, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.5/site-packages/blivet/populator/populator.py", line 451, in populate
    self._populate()
  File "/usr/lib/python3.5/site-packages/blivet/threads.py", line 45, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.5/site-packages/blivet/blivet.py", line 271, in reset
    self.devicetree.populate(cleanup_only=cleanup_only)
  File "/usr/lib/python3.5/site-packages/blivet/threads.py", line 45, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.5/site-packages/blivet/osinstall.py", line 1175, in storage_initialize
    storage.reset()
  File "/usr/lib64/python3.5/threading.py", line 862, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib64/python3.5/site-packages/pyanaconda/threads.py", line 251, in run
    threading.Thread.run(self, *args, **kwargs)
ValueError: device is already in tree

Additional info:
addons:         com_redhat_kdump, com_redhat_docker
cmdline:        /usr/bin/python3  /sbin/anaconda
cmdline_file:   BOOT_IMAGE=vmlinuz initrd=initrd.img inst.stage2=hd:LABEL=Fedora-S-dvd-x86_64-25 inst.sshd
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         4.8.6-300.fc25.x86_64
product:        Fedora
release:        Cannot get release name.
type:           anaconda
version:        25