node.js : Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client – [duplicate]

After i do one post, the second time i try it gives me this error.

My post trys to send a json file as response, after a file is edited, it works the first time, the second time i do a post without closing the server stops working. If i restart de server it works again.

app.post("/data3", upload.array("files"), uploadFiles);

function uploadFiles(req, res) {
    fs.watchFile(

      "./data/results.json",
      
      {
        bigint: false,
      
        persistent: true,
      
        interval: 4000,
      },
      (curr, prev) => {
        console.log("nThe file was edited");
      
        console.log("Previous Modified Time", prev.mtime);
        console.log("Current Modified Time", curr.mtime);
        
        res.json(fs.readFileSync("./data/results.json", "utf8"));

        console.log(
          "The contents of the current file are:",
          fs.readFileSync("./data/results.json", "utf8")
        );
      }
    );
}