SSTTestCase is run twice in python unittest testsuite?

It seems that SSTTestCase is executed twice when it's added in unittest testsuite.

I have a test:

from sst.actions import go_to, assert_title_contains
from sst import cases

class TestUbuntu(cases.SSTTestCase):

    def test_homepages(self):
        go_to('http://www.ubuntu.com')
        assert_title_contains('Ubuntu')

Which is added to following suite (testscenarios is used for multiple browser support in case of other browsers than Firefox):

import unittest
import xmlrunner
import testtools
from testscenarios import generate_scenarios
from sst import cases

from test_ubuntu import TestUbuntu

try:
    from StringIO import StringIO
except ImportError:
    from io import StringIO

suite = unittest.TestSuite()
runner = xmlrunner.XMLTestRunner(stream=StringIO())

mytests = unittest.TestSuite()
mytests.addTest(TestUbuntu('test_homepages'))

for test in testtools.iterate_tests(mytests):
        test.scenarios = [('Firefox', dict(browser_factory=cases.browsers.FirefoxFactory(), xserver_headless=True, screenshots_on=True))]

suite.addTests(generate_scenarios(mytests))
runner.run(suite)

When the suite is being executed (python suite.py) the test is run twice. If I don't add the test to mytests, the test is executed once, but XML test result report is not being produced.

Is this a bug or feature? Should I be executing SSTTestCases only via SST-run?