Fixed a bug where stdout and stderr were logged twice by junitxml.

changelog/3491.bugfix.rst

@@ -0,0 +1 @@
+Fixed a bug where stdout and stderr were logged twice by junitxml.

testing/test_junitxml.py

@@ -458,6 +458,23 @@ class TestPython(object):
         fnode.assert_attr(message="expected test failure")
         # assert "ValueError" in fnode.toxml()
 
+    def test_xfail_captures_output_once(self, testdir):
+        testdir.makepyfile("""
+            import sys
+            import pytest
+
+            @pytest.mark.xfail()
+            def test_fail():
+                sys.stdout.write('XFAIL This is stdout')
+                sys.stderr.write('XFAIL This is stderr')
+                assert 0
+        """)
+        result, dom = runandparse(testdir)
+        node = dom.find_first_by_tag("testsuite")
+        tnode = node.find_first_by_tag("testcase")
+        assert len(tnode.find_by_tag('system-err')) == 1
+        assert len(tnode.find_by_tag('system-out')) == 1
+    
     def test_xfailure_xpass(self, testdir):
         testdir.makepyfile("""
             import pytest