Gets duplicate packages in a yarn.lock file

yarn-lock-duplicates.mjs

import("node:fs")
  .then(({ readFileSync, writeFileSync }) => {
    const packages = new Map();
    const output = [];
    readFileSync("yarn.lock")
      .toString()
      .split("\n")
      .forEach((l) => {
        if (l[0] === '"') {
          const [pkg] = l.split("@npm:");
          if (packages.has(pkg)) {
            if (packages.get(pkg)) {
              output.push(packages.get(pkg));
              packages.set(pkg, false);
            }
            output.push(l);
          } else {
            packages.set(pkg, l);
          }
        }
      });
    writeFileSync("yarn.lock.dup", output.join("\n"));
  })
  .catch((e) => console.error(e));