fix(python): avoid duplicate captured argument in hoisted guards (#4610)

[dbrattli]

Problem

Fixes #4610. A pattern-match guard that reuses its bound value generates invalid Python with a duplicate argument:

let mkTag (tag: string option) =
    match tag with
    | None -> ""
    | Some s when s.StartsWith("!") && not (s.StartsWith("!!")) -> s + " "
    | Some s -> "!<" + s + "> "

def _arrow0(tag: str, tag: Any=tag) -> bool:  # SyntaxError: duplicate argument 'tag'

This regressed in 5.0.0 (worked in 5.0.0-alpha.21).

Root cause

When a guard reuses its binding, it is hoisted into a helper function that already takes the scrutinee as its own parameter (tag). Fable's tail-call optimization in transformFunction appends the enclosing function's arguments as default-valued capture parameters (x: Any=x) to nested functions — and it appended tag again, even though the helper already declares it.

Fix

Skip a TCO capture parameter when the function already declares an argument with the same name. The local argument shadows the outer one, so the capture is both redundant and a Python syntax error.

Testing

• Added test guard reusing binding does not duplicate captured argument to tests/Python/TestPatternMatch.fs using the exact repro from the issue.
• Full Python test suite passes (2349 tests).

🤖 Generated with Claude Code

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded