var foo = new int[]{1,1,4,5,6,2,3,3,1,7,2,4,2,3};

IEnumerable<int> Bar(IEnumerable<int> seed, IEnumerable<int> currentLevel = null, int? level = null)
{
    currentLevel = currentLevel ?? Enumerable.Empty<int>();
    level = level ?? default(int);

    var result = seed
        .OrderBy(_ => _)
        .GroupBy(_ => _)
        .Select(_ => _.Skip(level.Value).Take(1))
        .SelectMany(_ => _);

    if (!result.Any())
    {
        return currentLevel;
    }

    currentLevel = currentLevel.Concat(result);

    return Bar(seed, currentLevel, level + 1);
}

Bar(foo).Dump();