Find / Count Redundant Records in a List<T> - linq

Question

I am looking for a way to identify duplicate records...only I want / expect to see them.
So the records aren't duplicated completely but the unique fields I am unconcerned with at this point. I just want to see if they have made X# payments of the exact same amount, via the exact same card, to the exact same person. (Bogus example just to illustrate)
The collection is a List<> further whatever X# is the List<>.Count will be X#. In other words all the records in the list match (again just the fields I am concerned with) or I will reject it.
The best I can come up with is to take the first record get value of say PayAmount and LINQ the other two to see if they have the same PayAmount value. Repeat for all fields to be matched. This seems horribly inefficient but I am not smart enough to think of a better way.
So any thoughts, ideas, pointers would be greatly appreciated.
JB

Answer 1

Something like this should do it.
var duplicates = list.GroupBy(x => new { x.Amount, x.CardNumber, x.PersonName })
.Where(x => x.Count() > 1);

Answer 2

Working example:
class Program
{
static void Main(string[] args)
{
List<Entry> table = new List<Entry>();
var dup1 = new Entry
{
Name = "David",
CardNumber = 123456789,
PaymentAmount = 70.00M
};
var dup2 = new Entry
{
Name = "Daniel",
CardNumber = 987654321,
PaymentAmount = 45.00M
};
//3 duplicates
table.Add(dup1);
table.Add(dup1);
table.Add(dup1);
//2 duplicates
table.Add(dup2);
table.Add(dup2);
//Find duplicates query
var query = from p in table
group p by new { p.Name, p.CardNumber, p.PaymentAmount } into g
where g.Count() > 1
select new
{
name = g.Key.Name,
cardNumber = g.Key.CardNumber,
amount = g.Key.PaymentAmount,
count = g.Count()
};
foreach (var item in query)
{
Console.WriteLine("{0}, {1}, {2}, {3}", item.name, item.cardNumber, item.amount, item.count);
}
Console.ReadKey();
}
}
public class Entry
{
public string Name { get; set; }
public int CardNumber { get; set; }
public decimal PaymentAmount { get; set; }
}
The meat of which is this:
var query = from p in table
group p by new { p.Name, p.CardNumber, p.PaymentAmount } into g
where g.Count() > 1
select new
{
name = g.Key.Name,
cardNumber = g.Key.CardNumber,
amount = g.Key.PaymentAmount,
count = g.Count()
};
You're unique entries are based off of the 3 criteria of Name, Card Number, and Payment Amount so you group by them and then use .Count() to count how many of those unique values exist. where g.Count() > 1 filters the group to duplicates only.