Nested Array in JSON: Push / Unshift with date object creates double entry

I'm experiencing a weird behaviour with JavaScript. I am trying to construct a JS object like this in JavaScript (node.js/express)

var docAudit = {date: nowObj, changed: {}};
docAudit.changed['N5'] = 'DieterDorian';

console.log(docAudit) prints out now as expected:

{ date: Thu Sep 04 2014 11:00:00 GMT+0200 (CEST),
  changed: { N5: 'DieterDorian' } }

Now I want to push or unshift this docAudit object into my 'historie' Array in my 'ip' JSON (fetched from mongoDB)

ip: {
    __v: 15
    _id: "53fb42bf52b6542527cb7d23"
    aktuell: {
        N5: "HansHerschel"
        Auftragsnr: "2018"
    }
    historie: [
       {
           date: "2014-09-04T09:53:20.533Z"
           changed: {
           N5: "HansHerschel"
       }
    ]
}

Now this operation:

ip.historie.unshift(docAudit);

should result in adding the docAudit object in front of the historie array inside the 'ip' JSON object. However what happens is this:

    ip: {
    __v: 16
    _id: "53fb42bf52b6542527cb7d23"
    aktuell: {
          // not important what happens here
    }
    historie: [
       {
           date: "2014-09-04T11:00:00.533Z"
           changed: {
           N5: "DieterDorian"
       },
       {
           date: "2014-09-04T11:00:00.533Z"
       },
       {
           date: "2014-09-04T09:53:20.533Z"
           changed: {
           N5: "HansHerschel"
       }
    ]
}

So what happens is an additional Date object gets pushed into the 'historie' array. I am thinking I might constructing the 'docAudit' object wrongly. Expected output is just to push the docAudit into the array without an additional date object.

Or if I print out the 'ip' object with console.log(ip):

{ changed: {}, date: Thu Sep 04 2014 11:48:51 GMT+0200 (CEST) },
     { date: Thu Sep 04 2014 11:48:51 GMT+0200 (CEST),
       changed: [Object] } ] }

This is my routing code:

  updateIP = function(req, res) {

    return IP.findById(req.params.id, function(err, ip) {
      if(!ip) {
        res.statusCode = 404;
        return res.send({ error: 'Not found' });
      }

      var docAudit = {'changed':{}, 'date': nowObj};
      docAudit.changed.N5 = 'DieterDorian'    
      ip.historie.unshift(docAudit);   

      ip.markModified('aktuell');
      ip.markModified('historie'); 

      return ip.save(function(err) {
        if(!err) {
          console.log('Updated');
          return res.send({ status: 'OK', ip:ip });
        } else {
          if(err.name == 'ValidationError') {
            res.statusCode = 400;
            res.send({ error: 'Validation error' });
          } else {
            res.statusCode = 500;
            res.send({ error: 'Server error' });
          }
          console.log('Internal error(%d): %s',res.statusCode,err.message);
        }
        res.send(ip);
      });
    });
  };

Update:

I think the question is whether the following broken-down code is correct:

 updateIP = function(req, res) {
     return IP.findById(req.params.id, function(err, ip) {

         // alter the ip JSON here...

         ip.save(function(err) {
             if(!err) {
                 return res.send({ status: 'OK', ip:ip });
             }
         }
     });
 }