What's the recommended place to perform validation: ViewModel, Model or Controller?

I have a registration page and would like to perform some validation (in addition to the StringLength and Required annotations on my ViewModel) for duplicate usernames and email addresses. Currently I perform this validation in my controller when the registration form is posted back. I'm not sure if this is the right place to do it though.

I can't imagine the ViewModel to be the right place as it would require the ViewModel to have a reference to my UserRepository. Does it make sense to have this kind of validation in the model classes?

If so, how do I implement this on the model so I can check if the information is valid before I sent it into my repository?

Update

Code of my controller action:

if (ModelState.IsValid)
        {
            if (!_userRepository.Exists(registerViewModel.Username))
            {
                if (!_userRepository.EmailExists(registerViewModel.Email))
                {
                    _userRepository.Add(
                        new User
                            {
                                Created = DateTime.Now, 
                                Email = registerViewModel.Email, 
                                Password = registerViewModel.Password, 
                                Username = registerViewModel.Username
                            });

                    _userRepository.SaveChanges();
                    TempData["registrationDetails"] = registerViewModel;

                    return RedirectToAction("Confirm");
                }
                else
                {
                    ModelState.AddModelError(string.Empty, "This email address is already in use.");
                }
            }
            else
            {
                ModelState.AddModelError(string.Empty, "This username is already taken.");
            }
        }

        return View(registerViewModel);
    }

Update 2

Should the domain model care about such constraints as duplicate user names or email addresses or is this something that the controller layer should worry about?

Update 3

It seems that putting the validation logic in the controller makes the most sense as it can be reused in remote validation and in model validation on submit. Is something like checking for duplicates generally a thing that should be done in controllers or does it make sense to have these kind of checks in a domain model?

Thanks,

I would suggest that you do this in Controllers.

The main reason is that whatever the structure of the app be, you would definitely need to use Ajax to notify the user whether the username has already been taken or not. Otherwise it's just bad usability which doesn't justify your code structure.

And for that you would like to have a action method which could ajax-ly see if the username exists or not.

Overall, it does mean you may end up with two sets of validation method (UI and model) but its all for a good cause.

Also to make a good usable site you would definitely be using a Javascript framework like knockoutjs or backbone. Thats true MVVM and in that case having your ViewModels being the Model layer classes (as Chris mentioned) is not a good idea. So basically you'll end up with two sets of validation anyways.