Error in converting viewstate to var ASP.NET C# - Erro same key has already been added

 ViewState["dt2"] = 

                 allData.Where(a => dtOpenVacancy.AsEnumerable().Select(q => q.Field<string>("disciplinename")).Contains(a.Field<string>("discipline")) && dtOpenVacancy.AsEnumerable().Select(q => q.Field<string>("institutename")).Contains(a.Field<string>("institute")) && dtOpenVacancy.AsEnumerable().Select(q => q.Field<int>("cadreid")).Contains(int.Parse(ddlCadreType.SelectedItem.Value))).CopyToDataTable()