Опитах по този начин и се получи
db.UserSchema = new db.Schema({
userName: {type: String, index: {unique: true, dropDups: true}},
emailId: {type: String, index: {unique: true, dropDups: true}},
password: {type:String,
required:true},
contactNumber: String,
region: String,
isActive:Boolean,
lastLoginDateTime:Date
});