[6londe]

Repeated Connection field lines are currently overwritten during both the server's request validation and the client's 101-response validation. A valid handshake containing:

Connection: Upgrade
Connection: keep-alive

raises RemoteProtocolError: Missing header, 'Connection: Upgrade' because only the final field's tokens are checked.

Accumulate tokens from every field line, using the existing comma-list parser and case-insensitive Upgrade check. Missing headers and lists without Upgrade continue to be rejected. This follows combined field-value handling in RFC 9110 §5.2–5.3 and the WebSocket handshake token requirement. The change is based directly on current main and is independent of #210.

Validation on macOS arm64:

• Original main: 230 tests passed. New regression on unchanged main: 8 failed, 16 controls passed.
• Python 3.10.20 and 3.12.13: all 254 tests passed (python -bb -m pytest; on 3.12 also --cov=wsproto --cov-report=term --cov-report=xml). Total line/branch coverage stays at 96.57%; all changed executable lines are covered.
• An independent raw-socket peer over loopback TCP improved from 16/24 to 24/24 passing flows: both endpoint roles, token positions, mixed casing, comma lists, chunked reads, and missing-token/header rejection controls. Successful handshakes exchange a 640-byte PCM-shaped binary payload.
• mypy src/ (strict), Ruff check/format for the new test, make -C docs clean html, check-manifest, python -m build --outdir dist/, and twine check dist/* passed.

Other Python/platform combinations await upstream CI. Development groups were installed using uv pip install -e . --group dev in Python 3.12, avoiding the existing universal resolver conflict between Python 3.10 support and the Sphinx 8 documentation group; no dependency files changed.

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