using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var originalList = new List<Selection>()

        {

            new Selection() {IdSelectionType = "1", Name = "Under (75.50)", Visible = true},

            new Selection() {IdSelectionType = "2", Name = "Under (75.50)", Visible = true},

            new Selection() {IdSelectionType = "1", Name = "Under (75.50)", OddValue = 1.840m, Visible = true},

            new Selection() {IdSelectionType = "2", Name = "Under (75.50)", OddValue = 1.860m, Visible = true},

        };

​

        var noneNulls = originalList.Where(ol => ol.OddValue != null);

​

        var endList = originalList

            .Where(ol => !noneNulls.Select(nn => nn.IdSelectionType).Contains(ol.IdSelectionType))

            .Union(noneNulls);

        endList.Dump();

    }

}

​

public class Selection

{

    public string IdSelectionType {get; set;}

    public string Name {get; set;}

    public decimal? OddValue {get; set;}

    public bool Visible {get; set;}

}