How to get duplicate items for specific Ids?

Question

How do I get the list of repeating items with the same fpId and eclId?

var lst = (from fp in FPIds
        from et in ETIds
        from ec in ECIds
        join eclp in ECLP on ec equals eclp.ecId
        join ecl in ECL on eclpId equals clId                                   
        select new  {
                fpId = fp,
                etId = et,
                ecId = ec,
                eclId  
        }).ToList();

Answer 1

I managed to solve this way, posted as an answer, because it can help other people. Thank you!

if (lst.GroupBy(p => new { p.fpId, p.eclId })
       .Where(x => x.Count() > 1)
       .Sum(x => x.Count()) == 0){

                }