Unique key in embedded document

I have the following schemas:

In User.js

var User = new schema({
  'name': {type: String},
  'email': {type: String},
  'gender': {type: String},
  'password': String,
  'salt': String,
});

//ensure name is unique
//The reason why I did not use index:{unique:true} is because it return
//ugly error message (err: 'E11000 duplicate key error index: test.users.$name_1  
//dup key: { : "test1" }')
User.path('name').validate(function(name, respond){
  var UserModel = mongoose.model("User");
  UserModel.findOne({name: name}, function(e, user){
    if(user) respond(false);
    else respond(true);
  });
}, 'Username Already in Use');

In Discussion.js

var Discussion = new schema({
  'topic': {type: String},
  'description': {type: String},
  'datetime': {type: Date},
  'status': String,
  'user': [UserSchema]
});

When I save a new discussion with user that already existed in db, I get a "Username Already in Use" error message. A new user should have a unique name, while a new discussion should have an existing user. How can I overcome this issue?

User.findOne({name: name}, function(err, user){
  var discussion = new Discussion();
  discussion.user.push(user);
  discussion.save();
});