Exception groups raised from another exception print their cause chain twice

[SemTiOne]

Found while working with #14781.

When an ExceptionGroup is raised from another exception, the cause is printed twice in the traceback output. This is the same root cause as #8321, but the BaseExceptionGroup branch in repr_excinfo() was missed.

Minimal example:

def test_repro():
    try:
        raise RuntimeError("original cause")
    except RuntimeError as exc:
        raise ExceptionGroup("group", [ValueError("inner")]) from exc

RuntimeError: original cause and the "The above exception was the direct cause" line appear twice: once from traceback.format_exception() (see #9159) walking __cause__ itself, and once more from pytest's chain-walking loop.

Environment

• 9.2.0.dev117+g2886fc64b.d20260729
• Python 3.14.3
• Windows

Packages

• attrs: 25.4.0
• exceptiongroup: 1.3.1
• iniconfig: 2.3.0
• packaging: 26.0
• pluggy: 1.6.0
• pytest: 9.2.0.dev117+g2886fc64b.d20260729