mongoose - Node.Js Mean Stack, Save method always return "User Already Exists" -

i learning nodejs mean stack , working mean.js sample app, works fine , able create first user sign form. creating user give me "user name alreay exists" message. i've applied check on code before save see if user exists ore not , mongoose findone method returns me null(and right because user provided username not exist) save method still gives error, user exist

here sign code:

/**  * signup  */ exports.signup = function(req, res) {     // security measurement remove roles req.body object     delete req.body.roles;      // init variables     var user = new user(req.body);      var message = null;      // add missing user fields     user.provider = 'local';     user.displayname = user.firstname + ' ' + user.lastname;      user.findone({         "username": user.username     }, function(oerr, dbuser) {         if (oerr) {             return res.status(400).send({                 message: errorhandler.geterrormessage(err) + ' user name' + user.username             });         }         if (dbuser === null) {             // save user             user.save(function(err) {                 if (err) {                     return res.status(400).send({                         message: errorhandler.geterrormessage(err) + ' user name' + user.username                     });                 } else {                     // remove sensitive data before login                     user.password = undefined;                     user.salt = undefined;                      req.login(user, function(err) {                         if (err) {                             res.status(400).send(err);                         } else {                             res.json(user);                         }                     });                 }             });         } else {             return res.status(400).send({                 message: "this user exist buddy. " + ' user name' + user.username             });         }     }); }; 

thanks