Easiest way to copy/clone a mongoose document instance?

Can you clarify what you mean by "copy/clone"? Are you going trying to create a duplicate document in the database? Or are you just trying to have two vars in your program that have duplicate data?

If you just do:

Model.findById(yourid).exec(
    function(err, doc) { 
        var x = doc; 
        Model.findById(yourid).exec(
            function(err, doc2) {
                var y = doc2;
                // right now, x.name and y.name are the same
                x.name = "name_x";
                y.name = "name_y";
                console.log(x.name); // prints "name_x"
                console.log(y.name); // prints "name_y"
            }); 
    });

In this case, x and y will be two "copies" of the same document within your program.

Alternatively, if you wanted to insert a new copy of the doc into the database (though with a different _id I assume), that would look like this:

Model.findById(yourid).exec(
    function(err, doc) {
        var d1 = doc;
        d1._id = /* set a new _id here */;
        d1.isNew = true;
        d1.save(callback);
    }
);

Or if you're doing this from the outset, aka you created some document d1, you can just call save twice without setting the _id:

var d1 = new Model({ name: "John Doe", age: 54 });
d1.save(callback);
d1.save(callback);

There will now be two documents with differing _id's and all other fields identical in your database.

Does this clarify things a bit?

You need to reset d1.isNew = true; as in:

Model.findById(yourid).exec(
    function(err, doc) {
        doc._id = mongoose.Types.ObjectId();
        doc.isNew = true; //<--------------------IMPORTANT
        doc.save(callback);
    }
);