❔ how remove duplicates from string?

private IEnumerable<(string Title, string Date)> GetFormattedHolidays(CalendarDto calendarDto)
    {
        foreach (var holiday in calendarDto.Holidays)
        {
            foreach (var day in calendarDto.Days)
            {
                if (day.Holiday == holiday.Id.ToString())
                {
                    var date = day.Day?.ToString("dd MMMM", _context.Culture);

                    yield return ($"{holiday.Title}", $"{date}");
                }
            }
        }
    }