Object moves instead of added

I am stumped on this issue. The behavior I'm trying to achieve is one test to have multiple groups of learners allowed. This works correctly, but when I try to assign the same group for multiple tests, it will be moved rather than added. I think the photographs explain this best.

Here are my models:

public class TestParameters
{
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }

    [DefaultValue(60)]
    //in minutes
    //0 = no limit
    [DisplayName("Time Limit")]
    public int TimeLimit { get; set; }
    [Required]
    [DefaultValue(10)]
    [DisplayName("Number of Questions")]
    public int NumQuestions { get; set; }

    [Required]
    [DisplayName("Open From")]
    public DateTime OpenFrom { get; set; }
    [DisplayName("Open To")]
    public DateTime OpenTo { get; set; }
    [DisplayName("Allowed Groups")]
    public virtual ICollection<StudentGroup> AllowedGroups { get; set; } 
}

public class StudentGroup
{
    [Required]
    [Key]
    public string Code { get; set; }

    [Required]
    public string Name { get; set; }
    public string Description { get; set; }
}

      

        
        
        
      

    

And the corresponding controller:

[HttpPost]
public ActionResult NewGroup()
{
    var groupCode = Request["groupCode"];
    var paramId = Request["parametersId"];

    var group = db.StudentGroups.Find(groupCode);
    var parameters = db.TestParameters.Find(int.Parse(paramId));

    if (group == null)
    {
        ViewBag.ErrorMessage = "No group with code " + groupCode + " exists.";
    }
    else if (parameters.AllowedGroups.Contains(group))
    {
        ViewBag.ErrorMessage = "The group with code " + groupCode + " already has access to this test.";
    }
    else
    {
        parameters.AllowedGroups.Add(group);
        db.SaveChanges();
    }
    return View(parameters);
}

      

        
        
        
      

    

What could be causing this behavior?