Remove duplicacy in linq c# .

I have an issue. How to remove duplicate record from list. It's me code please check.

public class SupplementResponse
{
public int Response { get; set; }
public string ErrorMessage { get; set; }
public List lst_Supplement { get; set; }
public List lstOf_Supplement { get; set; }
}
public class SupplementDetail
{
public int SupplementsId { get; set; }
public string SupplementType { get; set; }
public string SupplementPDF { get; set; }
public int SupplementStatus { get; set; }
}
public class SupplementListDetail
{
public int SupplementsId { get; set; }
public string SupplementType { get; set; }
public string SupplementPDF { get; set; }
public int SupplementStatus { get; set; }
}
SupplementResponse obj_result = new SupplementResponse();
List lst = new List();
List list = new List();
var result = lst.Union(list).ToList(); //Face Issue here
var PDFURL = new ConfigurationBuilder().AddJsonFile("appsettings.json").Build().GetSection("AppSettings")["SupplementUrl"];
try
{
var matchingdata = (from c in _context.TblSupplementDetail
join sc in _context.SupplementMaster on c.SupplementId equals sc.SupplementId
where c.UserId == obj_parameter.UserId
select new
{
sc.SupplementId,
sc.SupplementType,
sc.SupplementPdf,
c.SupplementStatus
}).ToList();
if (matchingdata.Count > 0)
{
foreach (var item in matchingdata)
{
SupplementListDetail obj_supplement = new SupplementListDetail();
obj_supplement.SupplementsId = item.SupplementId;
obj_supplement.SupplementType = item.SupplementType;
obj_supplement.SupplementPDF = PDFURL + "" + item.SupplementPdf;
obj_supplement.SupplementStatus = Convert.ToInt32(item.SupplementStatus);
lst.Add(obj_supplement);
}
}
var Supplement_List = _context.SupplementMaster.ToList();
{
if(Supplement_List .Count >0)
{
foreach (var item in Supplement_List)
{
SupplementDetail obj_supplement = new SupplementDetail();
obj_supplement.SupplementsId = item.SupplementId;
obj_supplement.SupplementType = item.SupplementType;
obj_supplement.SupplementPDF = PDFURL + "" + item.SupplementPdf;
obj_supplement.SupplementStatus = Convert.ToInt32(item.SupplementStatus);
list.Add(obj_supplement);
}
}
}
obj_result.Response = 1;
obj_result.ErrorMessage = "No Error Found";
obj_result.lst_Supplement = lst;
obj_result.lstOf_Supplement = list;
}
catch (Exception ex)
{
obj_result.Response = 0;
obj_result.ErrorMessage = "Internal Server Error";
}
return obj_result;

it's my code. It's more important for me