swc can report the exact same fatal parse diagnostic more than once. deno_ast collects every fatal diagnostic into ParseDiagnosticsError, whose Display prints each one, so the identical error was shown twice:

$ deno
> for (console.log("a") of [1]);
error: SyntaxError: The left-hand side of an assignment expression must be a variable or a property access.
  |
1 | for (console.log("a") of [1]);
  |      ~~~~~~~~~~~~~~~~
    at file:///repl.tsx:1:6

SyntaxError: The left-hand side of an assignment expression must be a variable or a property access.
  |
1 | for (console.log("a") of [1]);
  |      ~~~~~~~~~~~~~~~~
    at file:///repl.tsx:1:6

The same duplication happens for deno run main.ts on that source, since both paths surface deno_ast::TranspileError.

This collapses identical diagnostics (ParseDiagnostic compares specifier + range + kind) while preserving order, at the point the transpile error is produced. Distinct diagnostics are still all reported.

Fixes #27804