What is the problem in express framework Error: Can't set headers after they are sent?

Question

Node.js

Hello, an error occurs when trying to form a request to vk api and issue a text file for download. As you can see, the file is rendered directly from the data acquisition function, which of course is wrong in terms of code organization, but at that time I just came up with it. An important note - the error occurs when you re-access the
index.js url

...
var vk = require('./lib/vk.js');

...

app.get('/vk', function(req, res) {

    if(req.query.action === 'getwall') {
      vk.getMessages('wall.get', {'owner_id' : req.query.id},res);
    } else {
        res.render('home', {view:'vk'});
    }
});

...
vk.js

exports.getMessages = function (req, params, res) {
    vk.setSecureRequests(true);
    vk.request(req, params);

    vk.on('done:wall.get', function (_o) {
        if(_o) {
        res.set({"Content-Disposition":"attachment; filename=\"backup.txt\""});
        res.send(JSON.stringify(_o, null, 4));
        //res.render('home', {vk: JSON.stringify(_o, null, 4)});
        }
    });
};

log output

5|index    | Error: Can't set headers after they are sent.
5|index    |     at ServerResponse.setHeader (_http_outgoing.js:358:11)
5|index    |     at ServerResponse.header (/../vk/node                                                                                            _modules/express/lib/response.js:719:10)
5|index    |     at ServerResponse.header (../vk/node                                                                                            _modules/express/lib/response.js:722:12)
5|index    |     at VK.<anonymous> (/.../vk/lib/vk.js:1                                                                                            5:6)
5|index    |     at emitOne (events.js:101:20)
5|index    |     at VK.emit (events.js:188:7)
5|index    |     at IncomingMessage.<anonymous> (/.../v                                                                                            k/node_modules/vksdk/sdk.js:307:31)
5|index    |     at emitNone (events.js:91:20)
5|index    |     at IncomingMessage.emit (events.js:185:7)
5|index    |     at endReadableNT (_stream_readable.js:974:12)

Modified the code, the error continues - an important remark, when you re-access the site.
index.js

app.get('/getwall/:action?/:id?',vk.getWall, function(req, res) {
        res.render('home', {view:'vk'});
});

vk.js

exports.getWall = function (req, res, next) {
  if(req.query.id)
    vk.setSecureRequests(true);
    vk.request('wall.get',{'owner_id' : req.query.id});
    vk.on('done:wall.get', function (_o) {
      console.log(req.query.id);
        if(_o) {
          req.wall =  {vk: JSON.stringify(_o, null, 4)};
          next();
        }
    });
};

Answer 1

If you do something res.send[.render, .end, etc.], then you don't need to send anything after, otherwise this error will occur.

Answer 2

You are subtly hinted that headers must be set before sending the main data. Like "minced meat cannot be turned back."