The C++ parser was not rejecting duplicate parameter names across
destructuring patterns in non-simple parameter lists. For example,
`function f({ bar, ...a }, { bar, ...b }) {}` was accepted despite
being a syntax error per spec.
The existing inline duplicate check only ran for identifier parameters,
missing the case where both parameters are binding patterns. Add a
post-parse pass that collects all bound names and checks for duplicates
when the parameter list is non-simple (or in strict mode/arrows).
Also fix existing tests that relied on the incorrect behavior and add
new test coverage for destructuring duplicate detection.
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