PasswordSignInAsync causes "The instance of entity type 'User' cannot be tracked because another instance with the key value" Why?

I'm, using ASP NET Core Identity in order to log in user into his account

But for some reason it started throwing me an Exception

InvalidOperationException: The instance of entity type 'User' cannot be tracked because another instance with the key value '{Id: 99d1aa12-3f75-497d-a4cc-5d2103c1b042}' is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached.

What's causing that?

public async Task<IActionResult> Login(string Login, string Password, string ReturnUrl = "/")
{
    var user = await _context.Users.AsNoTracking().FirstOrDefaultAsync(x => x.UserName == Login);

    if (user == null)
    {
        return View("Error");
    }

    var sign = await _signInManager.PasswordSignInAsync(user, Password, true, true); // here

    if(!sign.Succeeded)
    {
        return View("Error");
    }

    return Redirect(ReturnUrl);
}