Uniquify Names in a list of objects with a Name property, by appending _2, _3, etc.

gistfile1.txt

void Main()
{
    // Define the list
    var coll = new List<Foo>
    {
        new Foo { Name = "ross", Description = "me" },
        new Foo { Name = "joel", Description = "also me" },
        new Foo { Name = "ross", Description = "me #2" },
        new Foo { Name = "ross", Description = "me #3" },
        new Foo { Name = "joel", Description = "also me #2" }
    };

    // Rename duplicates by grouping and indexing
    var renamed = coll
        .GroupBy(c => c.Name.ToUpperInvariant())
        .SelectMany(g => g.Select((c, i) => new {
            Foo = c,
            NewName = i == 0 ? c.Name : $"{c.Name}{i + 1}"
        }));

    // rename each item that needs renaming
    foreach (var item in renamed)
    {
        item.Foo.Name = item.NewName;
    }

    // in LINQPad, Output the result
    coll.Dump("Renamed Collection");
    // if using in another app, you'd return the collection at this point.
}

// Class definition.  Could possibly be helpful for this to be an interface providing a Name property 
// and nothing else, to be used with any other class
public class Foo
{
    public string Name { get; set; }
    public string Description { get; set; }
}