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_capteesys_issue.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+import sys
+from typing import TYPE_CHECKING
+
+
+if TYPE_CHECKING:
+    from _pytest.capture import CaptureFixture
+    from _pytest.fixtures import SubRequest
+
+
+def test_dummy_test_with_traceback(
+    request: SubRequest, capteesys: CaptureFixture[str]
+) -> None:
+    print("Hello world stdout", flush=True)
+    print("Hello world stderr", file=sys.stderr, flush=True)