Try with below code:
var ChkDocumentData = new List<documentslists>()
{
new DocumentsLists {DocumentId = 42, DocumentName ="Passport"},
new DocumentsLists {DocumentId = 44, DocumentName ="Adhar Card"},
new DocumentsLists {DocumentId = 45, DocumentName ="Driving License"},
new DocumentsLists {DocumentId = 46, DocumentName ="Voter ICard"}
};
var AppDocData = new List<documentslists>()
{
new DocumentsLists {DocumentId = 44, DocumentName ="Adhar Card"}
};
List<documentslists> tempList = ChkDocumentData.Concat(AppDocData).ToList<documentslists>();
var idList = tempList.Select(x => new { x.DocumentId, x.DocumentName }).Distinct();
foreach(var temp in idList)
{
int DocumentId = temp.DocumentId;
string DocumentId = temp.DocumentName;
}
You can do it different ways(distinctby), here is one solution.