gistfile1.cs

//Prepare test data. Could be a set returned 
//from a database query
var list = new List<int>();
for (int i = 0; i < 1000000; i++)
{
    list.Add(i);
}
//Prepare a list to hold the results
//E.g list of all users born a certain year
var listOfInts = new List<IEnumerable<int>>();
for (int i = 0; i < 10; i++)
{
    //Select from the large list all users
    //that satisfy the criteria
    listOfInts.Add(list.Where(a => a == i));
}
//Now, loop through all years and select the
//first user for every year
foreach(var l in listOfInts)
{
    Console.WriteLine(l.First());
}