public JsonResult LoadOccupantsDetailedInformation()
{
    //Load the personsFormModel with data
    return new JsonpResult(personsFormModel, JsonRequestBehavior.AllowGet);
}

[HttpPost]
public ActionResult SaveOccupantsDetailedInformation(
PersonsFormModel personsFormModel)
    {
//This line is always returning true even if I pass 2 persons with the same ssn
        if (ModelState.IsValid == false)
        {
            var errorList = ModelState.ToDictionary(
                      kvp => kvp.Key,
                      kvp => kvp.Value.Errors.Select(e => e.ErrorMessage).ToArray()
                );
            return Json(new { Errors = errorList });
        }
        //Save the data in personsFormModel to database
        return Json(new JsonResultViewModel { Success = true });
    }


public partial class PersonsFormModel : List<PersonFormModel>, IValidatableObject
{
    public IEnumerable<System.ComponentModel.DataAnnotations.ValidationResult> Validate(
    ValidationContext validationContext)
    {
        var validationResults
            = new List<System.ComponentModel.DataAnnotations.ValidationResult>();
        if (this.SSNs.Count() > this.SSNs.Distinct().Count())
        {
            validationResults.Add(new System.ComponentModel.DataAnnotations.ValidationResult(
            "All the persons in the household should have a unique SSN\ITIN number",
                new[] { "SSN" }));
        }
        return validationResults;
    }
    private IEnumerable<string> SSNs
    {
        get
        {
            return this.Select(element => element.SSN);
        }
    }
}
    public class PrequalifyOccupantListPersonDetailedFormModel
{
    [Required(ErrorMessage = "SSN is required")]
    public string SSN { get; set; }
}

[Test]
        public void Should_validate_person_form_model()
        {
            var input = new PersonsFormModel();
            input.Add(new PersonFormModel { SSN = "33" });
            input.Add(new PersonFormModel { SSN = "33" });
            _controller.ViewData.ModelState.Clear();

            var results = new List<ValidationResult>();

            bool isValid = Validator.TryValidateObject(input,
                new ValidationContext(input, null, null),
                results,true);

            Assert.IsTrue(!isValid, "Cannot have duplicates.");
        }

***Controller:***
public JsonResult LoadOccupantsDetailedInformation()
{
    //Load the personsFormModel with data
    return new JsonpResult(personsFormModel, JsonRequestBehavior.AllowGet);
}

[HttpPost]
public ActionResult SaveOccupantsDetailedInformation(
PersonsFormModel personsFormModel)
    {
//This line is always returning true even if I pass 2 persons with the same ssn
        if (ModelState.IsValid == false)
        {
            var errorList = ModelState.ToDictionary(
                      kvp => kvp.Key,
                      kvp => kvp.Value.Errors.Select(e => e.ErrorMessage).ToArray()
                );
            return Json(new { Errors = errorList });
        }
        //Save the data in personsFormModel to database
        return Json(new JsonResultViewModel { Success = true });
    }

***Model:***
public class PersonsFormModel : IValidatableObject
{
    public PersonsFormModel()
    {
        this.Instance = new List<PersonFormModel>();
    }

    public List<PrequalifyOccupantListPersonFormModel> Instance { get; set; }

    public void Add(PersonFormModel personFormModel)
    {
        Instance.Add(personFormModel);
    }

    public IEnumerable<System.ComponentModel.DataAnnotations.ValidationResult> Validate(
    ValidationContext validationContext)
    {
        var validationResults
            = new List<System.ComponentModel.DataAnnotations.ValidationResult>();
        if (this.SSNs.Count() > this.SSNs.Distinct().Count())
        {
            validationResults.Add(new System.ComponentModel.DataAnnotations.ValidationResult(
            "All the persons in the household should have a unique SSN\ITIN number",
                new[] { "SSN" }));
        }
        return validationResults;
    }
    private IEnumerable<string> SSNs
    {
        get
        {
            return Instance.Select(element => element.SSN);
        }
    }
}

public class PersonFormModel
{
    [Required(ErrorMessage = "SSN is required")]
    public string SSN { get; set; }
}

View:

// Note, I am passing the "Instance" here.

 var postData = JSON.stringify({ Instance: list });

            RealPage.DataManager.POST({
                url: "/Approval/SaveOccupantsDetailedInformation"
                , data: postData
                , success: function (data) {
                    if (data && data.Success) {

                    }
                }
            });