C#

string[] lines = File.ReadAllLines(filePath);
var barcodes = lines.Select(line => {string[] parts = line.Split(','); 
                                     return parts[2]; });

var x = barcodes.GroupBy(b => b);

string[] lines = File.ReadAllLines(@"C:\Users\<user>\Desktop\Test data.txt");
var barcodes = lines.Select(line => {string[] parts = line.Split(',');
return parts[2]; });
var x = barcodes.GroupBy(b => b);

using System;
using System.Collections.Generic;
using System.Windows;
using System.IO;
using System.Linq;

namespace Inventory
{
	/// <summary>
	/// Interaction logic for Window1.xaml
	/// </summary>
	public partial class Window1 : Window
	{
		void button1_Click(object sender, RoutedEventArgs e)
		{
			
			string[] lines = File.ReadAllLines(@"C:\Users\sdadmin\Desktop\Test data.txt");
			var barcodes = lines.Select(line => {string[] parts = line.Split(',');
			                            	return parts[2]; });
			var x = barcodes.GroupBy(b => b);		
		}
		
		public class List
		{
			public string Date{get; set;}
			public string Time{get; set;}
			public string EAN {get; set;}
		}
	}
}

string[] lines = File.ReadAllLines(filePath);
var barcodes = lines.Select(line => {
    string[] parts = line.Split(',');
    return parts[2];
});
var x = barcodes.GroupBy(b => b);
foreach (var y in x)
    {
    Console.WriteLine($"{y.Key} : {y.Count()}");
    }

1578456329781 : 1
1578453697235 : 3
1548795358155 : 1

Quote:

I was thinking on using one loop to go threw the list, and then use the second loop to check for duplicates and count.

private readonly string bcdata = @"2020-07-08,13:54:16,1578456329781
2020-07-08,14:07:13,1578453697235
2020-07-08,14:08:03,1548795358155
2020-07-08,14:11:09,1578453697235
2020-07-08,14:11:42,1578453697235";

private char[] splitwhtspc = {'\r', '\n', ' ', '\t'};
private char[] splitcomma = {','};

// in some method or event-handler body:

var lines = bcdata.Split(splitwhtspc, StringSplitOptions.RemoveEmptyEntries);

Dictionary<DateTime, Int64> dttobc = lines.Select
(
    line => line.Split(splitcomma, StringSplitOptions.RemoveEmptyEntries)
).Select
(
    sary =>
        new
        {
            key = DateTime.Parse(sary[0] + " " + sary[1]),
            value = long.Parse(sary[2])
        }
)
.ToDictionary(
    kl => kl.key,
    vl => vl.value
);

Dictionary<long, int> dups = dttobc.Values.GroupBy(v => v).ToDictionary(g => g.Key, g => g.Count());