ASP.Net MVC Error: An item with the same key has already been added

Hi, 

I have 2 tables which is PlatformList and platform. I want to display all list from PlatformList and choose data and save it in platform table.

I tried using checkbox but it give me error.

An item with the same key has already been added

Can anyone help me since this is urgent?

DetailModel.cs (attribute in table Platform in db)

public string ph_no { get; set; }
public string platform_name { get; set; }
public int platform_id { get; set; }
public string baseType { get; set; }
public bool isSelected { get; set; }
public Nullable<int> order_id { get; set; }

PlatformList.cs

//attribute in PlatformList table in db
public string PlatformName { get; set; }
public string PH { get; set; }
public string BaseType { get; set; }
public int list_id { get; set; }
public bool IsChecked { get; set; }
 
//attribute in Platform table in db (already change the name but still got the same error)
public string phno { get; set; }
public string platformname { get; set; }
public int platformid { get; set; }
public string baseTypes { get; set; }
public bool IsSelected { get; set; }

Thanks in advance :)