title: I am getting a duplicate key error index: when I am trying to save data in mongodb
body: <p>I have a schema :</p>
  
  <pre><code>  var RegisterInfoSchema= new Schema({
    Organization:String,
    NGOName:String,
    Acronym:String,
    Address:String,
    Province:String,
    District:String,
    Tehsil:String,
    Telephone_number:String,
    Website:String,
    Demographics:String,
    Username:{type:String ,index: {unique:true}},
    Password:String
    })
  
  exports.savePersonalInfo = function (req,res){
  console.log("savePersInfo CALLED");
  
  var receivedObj = new RegisterInfo({
      Organization:           req.body.regOrgType ,
      NGOName:                req.body.regName,
      Acronym:                req.body.regAcronym ,
      Address:                req.body.regAddress ,
      Province:               req.body.regProvince,
      District:               req.body.regDistrict,
      Tehsil:                 req.body.regTehsil ,
      Telephone_number:       req.body.regTelNo  ,
      Website:                req.body.regWebAddr,
      Demographics:           req.body.regDemographics,
      Username:               req.body.regUserName ,
      Password:               req.body.regPsw
        });
  
       receivedObj.save(function(err){
      console.log("inside Save ");
      if(err){                        
          console.log(err);
      }
      else{
          console.log("Saved!!");
          res.send("");
  
      }
  
     });
     }
  </code></pre>
  
  <p>There is indexing in Username
  When I am trying to save data using save() method then it gives the following error:</p>
  
  <p><strong>{ [MongoError: E11000 duplicate key error index: testdb.registerinfos.$username_1  dup key: { : null }]
    name: 'MongoError',
    err: 'E11000 duplicate key error index: testdb.registerinfos.$username_1  dup key: { : null }',
    code: 11000,
    n: 0,
    lastOp: 0,
    connectionId: 339527,
    ok: 1 }</strong></p>