RoleProvider IsUserInRole Called Twice

Question

I recently noticed that GetRolesForUser() is called twice when I try to access a controller action that uses my custom attribute. I'm new to Forms and trying to fully understand each piece. Is this typical behavior or am I missing something?

Here is my controller:

namespace Forms.Controllers
{
    public class AdminController : Controller
    {
        [SiteAuthorization(Roles = "Admin")]
        public ActionResult AddUser()
        {
            return View();
        }
    }
}

Here is my Authorize Attribute:

namespace Forms.Attributes
{
    public class SiteAuthorization : AuthorizeAttribute
    {
        protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
        {
            if (!filterContext.HttpContext.User.Identity.IsAuthenticated)
            {
                base.HandleUnauthorizedRequest(filterContext);
            }
            else if (!Roles.Split(',').Any(filterContext.HttpContext.User.IsInRole))
            {
                filterContext.Result = new ViewResult
                {
                    ViewName = "~/Views/Shared/Unauthorized.cshtml"
                };
            }
            else
            {
                base.HandleUnauthorizedRequest(filterContext);
            }
        }
    }
}

Here is my custom Role Provider:

namespace Forms.Providers
{
    public class SiteRoleProvider : RoleProvider
    {
        private UserRoleRepo _userRoleRepo { get; }

        public SiteRoleProvider()
        {
            _userRoleRepo = new UserRoleRepo(new SMDMContext());
        }

        // Always called twice (?)
        public override string[] GetRolesForUser(string username)
        {
            return _userRoleRepo.GetUserRolesByUserName(username).ToArray();
        }
    }
}