Code to reproduce the bug:

const async = require('async');

const i_return_a_promise = () => Promise.resolve("Hello");

const fn = async.asyncify(i_return_a_promise);

var _count = 0;
fn((err) => {
  console.log("CB CALL #%d", ++_count, err);
  throw new Error("D:");
});

This code will print out:

CB CALL #1 null
CB CALL #2 [Error: D:]

This is because .then on a promise will catch errors, and return a rejected promise, which would then be caught by the .catch code, calling the callback twice. This PR fixes that behavior by handling both the resolve case and the rejection case in the .then handler.