class Record
{
    public int Id { get; set; }
    public string Title { get; set; }
}

var primaryRecords = new List<Record>
                         {
                             new Record {Id = 1, Title = “Record1″},
                             new Record {Id = 1, Title = “Record1″},
                             new Record {Id = 2, Title = “Record2″},
                             new Record {Id = 3, Title = “Record3″}
                         };

var secondaryRecords = new List<Record>
                           {
                               new Record {Id = 1, Title = “Record1″},
                               new Record {Id = 4, Title = “Record4″},
                               new Record {Id = 5, Title = “Record5″}
                           };

using System.Collections.Generic;
using System.Linq;

// HashSet is used to eliminate duplicate records from primary record
var recordIds = new HashSet<int>(primaryRecords.Select(r => r.Id));

// remove records that also exist in recordsId
secondaryRecords.RemoveAll(r => recordIds.Contains(r.Id));