Fix warnings summary header appearing twice

Ref: https://github.com/pytest-dev/pytest/pull/4450#discussion_r236017645
Ref: https://github.com/pytest-dev/pytest/pull/4399

doc/en/warnings.rst

@@ -33,8 +33,6 @@ Running pytest now produces this output::
       $REGENDOC_TMPDIR/test_show_warnings.py:4: UserWarning: api v1, should use functions from v2
         warnings.warn(UserWarning("api v1, should use functions from v2"))
 
-    -- Docs: https://docs.pytest.org/en/latest/warnings.html
-    ========================= warnings summary (final) =========================
     -- Docs: https://docs.pytest.org/en/latest/warnings.html
     =================== 1 passed, 1 warnings in 0.12 seconds ===================
 
@@ -358,8 +356,6 @@ defines an ``__init__`` constructor, as this prevents the class from being insta
       $REGENDOC_TMPDIR/test_pytest_warnings.py:1: PytestWarning: cannot collect test class 'Test' because it has a __init__ constructor
         class Test:
 
-    -- Docs: https://docs.pytest.org/en/latest/warnings.html
-    ========================= warnings summary (final) =========================
     -- Docs: https://docs.pytest.org/en/latest/warnings.html
     1 warnings in 0.12 seconds
 

doc/en/writing_plugins.rst

@@ -422,8 +422,6 @@ additionally it is possible to copy examples for an example folder before runnin
       $REGENDOC_TMPDIR/test_example.py:4: PytestExperimentalApiWarning: testdir.copy_example is an experimental api that may change over time
         testdir.copy_example("test_example.py")
 
-    -- Docs: https://docs.pytest.org/en/latest/warnings.html
-    ========================= warnings summary (final) =========================
     -- Docs: https://docs.pytest.org/en/latest/warnings.html
     =================== 2 passed, 1 warnings in 0.12 seconds ===================
 

src/_pytest/terminal.py

@@ -734,6 +734,8 @@ class TerminalReporter(object):
             else:
                 warnings = all_warnings
             self._already_displayed_warnings = len(warnings)
+            if not warnings:
+                return
 
             grouped = itertools.groupby(
                 warnings, key=lambda wr: wr.get_location(self.config)

testing/test_terminal.py

@@ -1094,7 +1094,34 @@ def test_terminal_summary_warnings_are_displayed(testdir):
         ]
     )
     assert "None" not in result.stdout.str()
-    assert result.stdout.str().count("warning_from_test") == 1
+    stdout = result.stdout.str()
+    assert stdout.count("warning_from_test") == 1
+    assert stdout.count("=== warnings summary ") == 2
+
+
+@pytest.mark.filterwarnings("default")
+def test_terminal_summary_warnings_header_once(testdir):
+    testdir.makepyfile(
+        """
+        def test_failure():
+            import warnings
+            warnings.warn("warning_from_" + "test")
+            assert 0
+    """
+    )
+    result = testdir.runpytest("-ra")
+    result.stdout.fnmatch_lines(
+        [
+            "*= warnings summary =*",
+            "*warning_from_test*",
+            "*= short test summary info =*",
+            "*== 1 failed, 1 warnings in *",
+        ]
+    )
+    assert "None" not in result.stdout.str()
+    stdout = result.stdout.str()
+    assert stdout.count("warning_from_test") == 1
+    assert stdout.count("=== warnings summary ") == 1
 
 
 @pytest.mark.parametrize(