prevent capteesys from duplicating output when -s disables global capture

src/_pytest/capture.py

@@ -1049,14 +1049,20 @@ def test_output(capteesys):
             assert captured.out == "hello\n"
     """
     capman: CaptureManager = request.config.pluginmanager.getplugin("capturemanager")
-    capture_fixture = CaptureFixture(
-        SysCapture, request, config=dict(tee=True), _ispytest=True
-    )
-    capman.set_fixture(capture_fixture)
-    capture_fixture._start()
-    yield capture_fixture
-    capture_fixture.close()
-    capman.unset_fixture()
+    if capman.is_globally_capturing():
+        capture_fixture = CaptureFixture(
+            SysCapture, request, config=dict(tee=True), _ispytest=True
+        )
+        capman.set_fixture(capture_fixture)
+        capture_fixture._start()
+        yield capture_fixture
+        capture_fixture.close()
+        capman.unset_fixture()
+    else:
+        # capteesys does nothing when global capturing is disabled.
+        # This is so that the "tee" part of cap-tee-sys is not
+        # implemented without the "cap" part.
+        yield CaptureFixture(SysCapture, request, _ispytest=True)
 
 
 @fixture

testing/test_capture.py

@@ -478,6 +478,30 @@ def test_one(capteesys):
         result.stdout.fnmatch_lines(["sTdoUt", "sTdeRr"])  # no tee, just reported
         assert not result.stderr.lines
 
+    def test_capteesys_no_global_capture(self, pytester: Pytester) -> None:
+        """When global capture is disabled (-s), capteesys should not duplicate output.
+
+        It should pass output straight through (printed once) and capteesys.readouterr()
+        should return empty strings since no per-test capture is active.
+        """
+        p = pytester.makepyfile(
+            """\
+            import sys
+
+            def test_one(capteesys):
+                print("sTdoUt")
+                print("sTdeRr", file=sys.stderr)
+                out, err = capteesys.readouterr()
+                assert out == ""
+                assert err == ""
+            """
+        )
+        # Run with -s to disable global capture; ensure each line appears exactly once
+        result = pytester.runpytest_subprocess(p, "-s", "--quiet", "--quiet")
+        assert result.ret == ExitCode.OK
+        assert result.stdout.str().count("sTdoUt") == 1
+        assert result.stderr.str().count("sTdeRr") == 1
+
     def test_capsyscapfd(self, pytester: Pytester) -> None:
         p = pytester.makepyfile(
             """\