`duel` is a CLI tool that lets TypeScript projects generate correct dual‑package builds (both ESM and CommonJS) without extra config files. It infers the primary module system from the `package.json` `type`, emits the secondary build in a separate folder, rewrites imports/ globals as needed, and can auto‑generate `exports` entries—making it useful for library authors who need to ship Node.js packages that work in both module systems.