DDD Domain Model Complex Validation

I am working on rewriting my ASP.NET MVC app using the domain driven design priciples. I am trying to validate my User entity. So far I am able to validate basic rules (like the username and password being a non null/whitespace string). However one of the rules, I need to make sure that the username is unique. However I need access to the database inorder to do this, which means I would have to inject my IUserRepository into my User entity like so.

public class User
{
    private readonly IUserRepository _userRepository;
    public User(IUserRepository repo)
    {
        _userRepository = repo;
    }

    public override void Validate()
    {
        //Basic validation code
        if (string.IsNullOrEmpty(Username))
            throw new ValidationException("Username can not be a null or whitespace characters");
        if (string.IsNullOrEmpty(Password))
            throw new ValidationException("Password can not be a null or whitespace characters");

        //Complex validation code
        var user = _userRepository.GetUserByUsername(Username);
        if (user != null && user.id != id)
            throw new ValidationException("Username must be unique")
    }
}

However this seems ... well wrong. Making my entity depend on my repository seems like a bad idea (correct me if I am wrong). But having the validation code in the entity makes sense. Where is the best place to put complex validation code?