Hide range attribute error message when required attribute shows the error message in MVC

See more:

C#

.NET

MVC4

In my form there is multiple validation conditions that needs show according the user input.
Example: when id is exists in database the error shows "Id exists" and when it is empty it shows "Id field is required" and unit filed also shows "unit is required" . I like to hide Unit filed error message when Id exists shows.

Thanks in Advance!

What I have tried:

public class IdIsUniqueAttribute : ValidationAttribute
{
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{

string id = value as string;
if (string.IsNullOrEmpty(id))
return null;
var repo = Ioc.Resolve(typeof(IRepository

<BO.Models.Employee>)) as IRepository<BO.Models.Employee>;
            if (repo == null)
                return null;
           
            var result = repo.Where(l => l.Id == id);            

            foreach (var item in result)
            {
                return new ValidationResult("User Already Exists");   
            }                         
            return null;
        }