Can't send res.send back without getting errors

I must be missing something majorly important here

router.get("/", async (req, res, next) => {
  let posts = await loadPostsCollection();
  console.log(await posts.find({}).toArray());
  res.send(await posts.find().toArray());
});

async function loadPostsCollection() {
  const client = await mongodb.MongoClient.connect(uri, {
    // useNewUrlParser: true,
    useUnifiedTopology: true,
  });
  return client.db("stack_1").collection("posts");
}

why do i get headers already sent error?

Express js @latest
MongoDb Atlas free cluster
db with single item in a ‘posts’ collection
I must be missing something incredibly simple.

The console log works as intended…
I’m pretty sure all my async await calls are at the right place…

Help..?

EDIT : ERROR:

http://localhost:3000/api/posts: 2020-07-23T23:57:06+03:00
[ { _id: 5f19e338adcae71eca1c3658, name: 'liad', status: 'active' } ]
(node:7894) UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:536:11)
    at ServerResponse.header (/media/liad/Data/liad_dev/vue-express-mongo/node_modules/express/lib/response.js:771:10)
    at ServerResponse.send (/media/liad/Data/liad_dev/vue-express-mongo/node_modules/express/lib/response.js:170:12)
    at ServerResponse.json (/media/liad/Data/liad_dev/vue-express-mongo/node_modules/express/lib/response.js:267:15)
    at ServerResponse.send (/media/liad/Data/liad_dev/vue-express-mongo/node_modules/express/lib/response.js:158:21)
    at /media/liad/Data/liad_dev/vue-express-mongo/server/routes/api/posts.js:10:7
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:7894) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:7894) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Edit 2:

refactored everything to use mongoose

I get the same error !

  const post = new Post({
    title: req.body.title,
    desc: req.body.desc,
  });
  console.log(post);
  post
    .save()
    .then((response) => {
      res.json(response);
    })
    .catch((err) => {
      res.json({ message: err });
    });
});

let routerPath = "/api/members";
app.use((req, res, next) => {
  let routerPath = req.path;
  app.use(`${routerPath}`, require(`./routes${routerPath}`));
  next();
});

let routerPath = "/api/members";
app.use((req, res, next) => {
  let routerPath = req.path;
  next();
  app.use(`${routerPath}`, require(`./routes${routerPath}`));
  next();
});