private void removedoubles(List<int> exclude, List<int> listcopy)
{
    for (int j = 0; j < exclude.Count(); j++)
    {
        for (int i = 0; i < listcopy.Count(); i++)
        {
            if (listcopy[i] == exclude[j])
            {
                if (i % 2 == 0) // even
                {
                    //listcopy.RemoveRange(i, i + 1);
                    listcopy.RemoveAt(i);
                    listcopy.RemoveAt(i);
                    i = i - 1;
                }
                else //odd
                {
                    //listcopy.RemoveRange(i - 1, i);
                    listcopy.RemoveAt(i - 1);
                    listcopy.RemoveAt(i - 1);
                    i = i - 2;
                }
            }
        }
    }
}

private List<Tuple<int, int>> getWithoutDoubles(
              HashSet<int> exclude, List<Tuple<int, int>> original)
{
    return original.Where(xy => (!exclude.Contains(xy.Item1) &&
                                 !exclude.Contains(xy.Item2)))
           .ToList();
}

var a = new List<int> {1, 2, 3, 4, 5};

var b = new List<int> {1, 2, 3};

var c = (from i in a let found = b.Any(j => j == i) where !found select i).ToList();