prevent capteesys from duplicating output when -s disables global capture

[ron-42]

Summary

Using the capteesys fixture with -s (--capture=no) printed output twice, even if the fixture wasn’t used in the test body. This issue is tracked in [pytest-dev/pytest#13784](#13784) and is fixed by this PR.

Fixes: #13784

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Root Cause

capteesys always instantiated a tee-ing SysCapture, so when global capture was disabled (-s), output went directly to the terminal and was also duplicated through the tee path.

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Fix

Enable tee only when global capture is active. When global capture is disabled, capteesys yields a non-teeing fixture to avoid duplication.

Changes localized to: src/_pytest/capture.py (capteesys fixture)

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Tests

Added regression test: test_capteesys_issue.py

• Reproduces duplication with -s and validates the fix.

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Behavior Comparison

Before:
Running pytest -s with capteesys in the test signature duplicated stdout/stderr.

After:
pytest -s prints each line exactly once (no duplication).
With capture enabled, capteesys continues to tee and capture as expected.

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Backwards Compatibility

• No public API changes.
• Behavior now aligns with the selected global --capture mode.

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Affected Files

• src/_pytest/capture.py
• test_capteesys_issue.py

[bluetech]

This test doesn't seem to do anything. I suggest looking at other tests in testing/test_capture.py and adding a test there.