title: Promise.all returns double array
body: <p>In my node Router I have the following:</p>
  
  <pre><code>router.get("/single-base/:id", (req, res) =&gt; {
  Base.find({ _id: req.params.id })
      .then(bases =&gt; {
          let basefetches = [];
  
          for (let base of bases) {
              basefetches.push(BaseUser.find({ baseId: req.params.id }));
          }
          return Promise.all(basefetches).then(users =&gt;
              res.json(users.map(user =&gt; user.serialize()))
          );
      })
      .catch(err =&gt; {
          console.error(err);
          res.status(500).json({ message: "Internal server error" });
      });
  });
  </code></pre>
  
  <p>when console logging <code>return Promise.all(basefetches).then(users =&gt; console.log(users))</code> I receive:</p>
  
  <pre><code>[ [ { created: 2018-08-27T21:37:42.151Z,
    _id: 5b846f268cb4481264298f82,
    userId: 'tester2',
    baseId: 5b81528e1314da25bc498085,
    acceptedMembership: false,
    isCreator: false,
    __v: 0 } ] ]
  </code></pre>
  
  <p>I have the exact same method in another place, and it returns only an empty array (this array should be empty in the beginning as well). 
  Why/when/how is this pushing an array into another array, how do I solve this so that the <code>users.map</code> does not throw:</p>
  
  <pre><code>TypeError: Cannot read property 'push' of undefined
  at Base.find.then.bases
  </code></pre>
  
  <p><strong>EDIT</strong>: I believe this has to do with the <code>BaseUser.find({})</code> method. When console logging: <code>Promise.all(basefetches).then(users =&gt; console.log(users));</code> I get the following: </p>
  
  <pre><code>{ created: 2018-08-25T12:43:02.640Z,
    _id: 5b81528e1314da25bc498085,
    creatorId: 5b7efd4db3036a13601ad8d7,
    title: 'testBase1',
    __v: 0 }
  [ [ { created: 2018-08-27T21:37:42.151Z,
        _id: 5b846f268cb4481264298f82,
        userId: 'tester2',
        baseId: 5b81528e1314da25bc498085,
        acceptedMembership: false,
        isCreator: false,
        __v: 0 } ] ]
  </code></pre>
  
  <p>I </p>