Does this actually test the change? As far as I can tell 'abort' was emitted only once even before. Can we have a better test?

What do you mean? I call abort() twice and 'abort' is only emitted once...

Yes, that's my point. it works like this even without the changes proposed here, so it is not clear what this is testing.

'use strict';

const http = require('http');

const server = http.createServer(function(req, res) {
  res.flushHeaders();
});

server.listen(function() {
  const req = http.get({ port: server.address().port });

  req.on('response', function(res) {
    const dump = res._dump;

    res._dump = function() {
      console.log('dump');
      dump.apply(res, arguments);
    };

    req.abort();
    req.abort();
    req.abort();
    req.abort();
  });

  req.on('abort', function() {
    console.log('abort');
  });
});

'abort' is already emitted only once. The difference is that res._dump() is only called once now. Hope it makes sense.

Makes sense. Updated. Not pretty but I can't see any other way.

Does it make sense to make this a separate test rather than modifying the existing test? My concern is that the existing test is such a straightforward one and this kind of makes it less obvious what's going on. (There's no comment explaining why req.abort() is being called twice. But also wouldn't we want a test where it is only called once?)