Detecting duplicate records, selecting only first and counting with LINQ/C#

Question

I am looking for a little help with designing a query using C#/LINQ to meet the following requirements:

I have a list of companies:-

Id  Name                Email       Address

1   Company A         [email protected]       abc

2   Company B         [email protected]       abc

3   Company C         [email protected]       abc

4   Company D         [email protected]       abc

5   Company A         [email protected]       abc

My goal is to detect duplicate items based on two fields, in this example 'name' and 'email'.

Desired output is a list of customers shown below:

1. Duplicate customers shuold only be shown once
2. The quantity of similar records should be shown.

Desired duplicate list:-

Id  Qty Name        Email       Address

1   2   Company A       [email protected]     abc (Id/details of first)

2   1   Company B       [email protected]     abc

3   1   Company C       [email protected]     abc

4   1   Company D       [email protected]     abc

Answer 1

If you explicitly want to use the lowest-ID record in each set of duplicates, you could use

var duplicates = companies
    .GroupBy(c => new { c.Name, c.Email })
    .Select(g => new { Qty = g.Count(), First = g.OrderBy(c => c.Id).First() } )
    .Select(p => new
        {
            Id = p.First.Id,
            Qty = p.Qty,
            Name = p.First.Name,
            Email = p.First.Email,
            Address = p.First.Address
        });

If you don't care which record's values are used, or if your source is already sorted by ID (ascending), you can drop the OrderBy call.

Answer 2

from c in companies
group c by new { c.Name, c.Email } into g
select new
{
   Id = g.First().Id,
   Qty = g.Count(),
   Name = g.Key.Name,
   Email = g.Key.Email,
   Address = g.First().Address
};