title: when using fixtures with scope session and class , tests with scope class runs twice
body: <p>I have a below py.test program where i need to 2 fixtures, one with scope "session" and other with scope "class", the fixture with scope "class" takes fixture with "session" as one of it's argument. </p>
  
  <p>While running tests that uses scope as "class", the tests seems to run twice, </p>
  
  <p>Below is the code.</p>
  
  <pre><code>import pytest
  
  @pytest.fixture(scope="session")
  def session_fixture(request):
      data = ['hello world']
      return data
  
  @pytest.fixture(scope="class")
  def class_fixture(session_fixture, request):
      if hasattr(request.cls, 'test1'):
          request.cls().test1(session_fixture)
      return session_fixture
  
  class TestClass:
      def test1(self,class_fixture):
          print("Hello World")
  </code></pre>
  
  <p>When i run the test, it seems to print "hello world" twice. </p>
  
  <p>Ouput:</p>
  
  <pre><code>$ py.test test7.py -s
  =============== test session start============================
  platform linux2 -- Python 2.7.5 -- py-1.4.27 -- pytest-2.7.0
  rootdir: /root/fix2, inifile: 
  plugins: multihost
  collected 1 items 
  
  test7.py Hello World
  Hello World
  . 
  ================ 1 passed in 0.09 seconds ===================
  </code></pre>
  
  <p>In the above program if i use the fixture "session_fixture" directly instead of "class_fixture", i see that "Hello world" is printed only once. </p>
  
  <p>Any hints on how i could resolve the issue.</p>