Description of problem:

I'm not able to tag test cases with the 'RHEL6' tag.

Version-Release number of selected component (if applicable):
Nitrate 3.5.0

Steps to Reproduce:
TestCase.add_tag(46490, 'RHEL6')

Actual results:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.6/xmlrpclib.py", line 1199, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib64/python2.6/xmlrpclib.py", line 1489, in __request
    verbose=self.__verbose
  File "nitrate.py", line 120, in request_with_cookies
    return self._parse_response(h.getfile(), sock)
  File "/usr/lib64/python2.6/xmlrpclib.py", line 1392, in _parse_response
    return u.close()
  File "/usr/lib64/python2.6/xmlrpclib.py", line 838, in close
    raise Fault(**self._stack[0])
xmlrpclib.Fault: <Fault 1: "MultipleObjectsReturned: get() returned more than one TestTag -- it returned 2! Lookup parameters were {'name': u'RHEL6'}">

Additional info:
Tagging with RHEL5 works fine.