Fix capteesys producing doubled output with --capture=no

changelog/13784.bugfix.rst

@@ -0,0 +1 @@
+Fixed ``capteesys`` producing doubled output when used with ``--capture=no`` (``-s``).

src/_pytest/capture.py

@@ -944,7 +944,14 @@ def _start(self) -> None:
 
     def close(self) -> None:
         if self._capture is not None:
-            out, err = self._capture.pop_outerr_to_orig()
+            if self._config.get("tee"):
+                # When tee is enabled, output was already written to the
+                # original stream in real-time by TeeCaptureIO.  Using
+                # pop_outerr_to_orig() would write it a second time via
+                # writeorg(), causing doubled output (see #13784).
+                out, err = self._capture.readouterr()
+            else:
+                out, err = self._capture.pop_outerr_to_orig()
             self._captured_out += out
             self._captured_err += err
             self._capture.stop_capturing()

testing/test_capture.py

@@ -478,6 +478,26 @@ def test_one(capteesys):
         result.stdout.fnmatch_lines(["sTdoUt", "sTdeRr"])  # no tee, just reported
         assert not result.stderr.lines
 
+    def test_capteesys_no_double_output_with_capture_no(
+        self, pytester: Pytester
+    ) -> None:
+        """Capteesys with --capture=no should not produce doubled output (#13784)."""
+        p = pytester.makepyfile(
+            """\
+            def test_one(capteesys):
+                print("hello world stdout")
+                out, err = capteesys.readouterr()
+                assert out == "hello world stdout\\n"
+            """
+        )
+        result = pytester.runpytest(p, "--quiet", "--quiet", "-rN", "-s")
+        assert result.ret == ExitCode.OK
+        # "hello world stdout" should appear exactly once, not twice.
+        count = result.stdout.lines.count("hello world stdout")
+        assert count == 1, (
+            f"Expected 'hello world stdout' once, but found {count} times"
+        )
+
     def test_capsyscapfd(self, pytester: Pytester) -> None:
         p = pytester.makepyfile(
             """\