Description of problem:
When there is running provision of system and I try to provision another system with same IP address, it will result in "IP Addresses must be unique. This IP Address has already been recorded. Please choose a different IP Address." error message.

Version-Release number of selected component (if applicable):
cobbler-2.0.7-66.el6sat.noarch

How reproducible:
always

Steps to Reproduce:
1. Have 2 hosts with same IP addresses
2. Register both hosts to Satellite and allow provisioning entitlement
3. Schedule provisioning of the first host
4. Schedule provisioning of the second host and we get error message "IP Addresses must be unique. This IP Address has already been recorded. Please choose a different IP Address."

From /var/log/cobbler/cobbler.log we get traceback:

Wed Sep 14 08:26:38 2016 - INFO | Exception occured: <class 'cobbler.cexceptions.CX'>
Wed Sep 14 08:26:38 2016 - INFO | Exception value: 'IP address duplicated: 192.168.122.100'
Wed Sep 14 08:26:38 2016 - INFO | Exception Info:
  File "/usr/lib/python2.6/site-packages/cobbler/remote.py", line 1762, in _dispatch
    return method_handle(*params)
   File "/usr/lib/python2.6/site-packages/cobbler/remote.py", line 767, in modify_system
    return self.modify_item("system",object_id,attribute,arg,token)
   File "/usr/lib/python2.6/site-packages/cobbler/remote.py", line 760, in modify_item
    return method(arg)
   File "/usr/lib/python2.6/site-packages/cobbler/item_system.py", line 581, in modify_interface
    if field == "ipaddress"           : self.set_ip_address(value, interface)
   File "/usr/lib/python2.6/site-packages/cobbler/item_system.py", line 300, in set_ip_address
    raise CX("IP address duplicated: %s" % address)


Actual results:
Simultaneous provision of 2 hosts with same IP address doesn't work.

Expected results:
Simultaneous provisioning of 2 hosts with same IP address should work.