I am building a simple chat application using react, express, and socket.io.

Every time that client sends the message to the server, the server will broadcast the message back to every user. I have a problem when receiving a message back from the backend server.

Every time user receives a message, the useEffect will run twice instead of only once.

useEffect(() => {
    socket.on("broadcast_msg", (data) => {
      setMsg((list) => [...list, data]);
    });
  }, [socket]);

PS. The backend server emits back to the front-end only once. Thank you for every response :D

Whole Code

import React from "react";
import { useEffect } from "react";
import { useState } from "react";

function Chat(props) {
  const { socket, username, room } = props;
  const [input, setInput] = useState("");
  const [msg, setMsg] = useState([]);

  // send msg to server
  const send_msg = (e) => {
    if (input !== "") {
      socket.emit("send_msg", {
        room: room,
        author: username,
        message: input,
      });
    }
  };

  // listen to boardcast msg
  useEffect(() => {
    socket.on("broadcast_msg", (data) => {
      setMsg((list) => [...list, data]);
    });
  }, [socket]);

  return (
    <div className="chat">
      <div className="chat-header"></div>
      <div className="chat-body">
        {msg.map((data) => {
          return (
            <>
              <h1>{data.author}</h1>
              <h1>{data.room}</h1>
              <h1>{data.message}</h1>
            </>
          );
        })}
      </div>
      <div className="chat-footer">
        <input
          type="text"
          placeholder="Enter the message..."
          onChange={(e) => {
            setInput(e.target.value);
          }}
        />
        <button onClick={send_msg}>Send</button>
      </div>
    </div>
  );
}

export default Chat;