LINQ

    public static IEnumerable<char> Calculate(string stringValue)
    {
        return stringValue
            .GroupBy(i=>i)
            .Select(g=>new{Symbol=g.Key,Count=g.Count()})
            .OrderByDescending(i=>i.Count)
            .GroupBy(i=>i.Count)
            .First()
            .Select(g=>g.Symbol);
    }