How to handles validate with two unique mongoose

I'm beginner with mongoose and nodejs . I'm have two field unique in mongoose , how can I validate with it . Here is my code:

model

var userSchema = new Schema({
    local:{
        email:{type:String,required:true,unique:true},
        username:{type:String,required:true,unique:true},
        password:{type:String,required:true}
    }
});

router

User.findOne({'local.email':req.body.email},function(err,user){
        if(err) console.log(err)
        if(user){
            res.json({message:'Email already exists'})
            next()
        }
    });

    User.findOne({'local.username':req.body.username},function(err,user){
    if(err) console.log(err)
    if(user){
        res.json({message:'Username already exists'})
        next()
    }
    })

    newUser = new User;
    newUser.local.email = req.body.email;
    newUser.local.username = req.body.username;
    newUser.local.password=req.body.password;
    newUser.save(function(err){
        if(err) console.log(err)
        res.json('Register Success');
    });

Im try this code above , but unfortunately it not work . When first time , I try post unique email , It work , but after it , I getting error duplicate key error . Cant set header after they are send . I'm not sure with code in my router . Can anyone show me what should I do ?

via Thanh Tùng