List<SelectListItem> retmodel = await dbContext.Certifications
    .Where(c => brevettiChePuoEmettere.Contains(c.Id) && c.Attivo == 1)
    .OrderByDescending(c => c.Livello).ThenBy(c => c.Descrizione)
    .Select(c => new SelectListItem { Value = c.Id.ToString(), Text = c.Descrizione, Selected = c.Id == selected })
    .Distinct()
    .ToListAsync();

SELECT DISTINCT CAST(`t`.`Id` AS char) AS `Value`, `t`.`Descrizione` AS `Text`, `t`.`Id` = @__selected_1 AS `Selected`
      FROM `t_certifications` AS `t`
      WHERE `t`.`Id` IN (2, .........) AND (`t`.`Attivo` = 1)