Express.js Error: "Cannot set headers after they are sent to the client"

[Mercure28]

Why does Express give "Cannot set headers after they are sent to the client"?

[TatyOko28]

This happens when you try to send multiple responses in a single request.

Incorrect:

app.get("/test", (req, res) => {
    res.send("Hello");
    res.send("World");  // Error: Headers already sent
});

Correct: Return after sending a response

app.get("/test", (req, res) => {
    res.send("Hello");
    return;
});