Cannot get distinct values using a SelectListitem

The problem with your current code is that Distinct will use the default comparer for SelectListItem. You will need to provide a custom comparer like this:-

public class SelectListItemComparer : IEqualityComparer<SelectListItem>
    {
        public bool Equals(SelectListItem x, SelectListItem y)
        {
            return x.Text == y.Text && x.Value == y.Value;
        }

        public int GetHashCode(SelectListItem  item)
        {
            int hashText = item.Text == null ? 0 : item.Text.GetHashCode();
            int hashValue = item.Value == null ? 0 : item.Value.GetHashCode();
            return hashText ^ hashValue;
        }
    }

Then you can use it like this:-

IEnumerable<SelectListItem> ldidList = _db.TrafficHits.Select(c => new SelectListItem
            {
                Value = c.Id.ToString(),
                Text = c.ldid
            }).Distinct(new SelectListItemComparer());

You can use the group by, then you select the first element per group :

IEnumerable<SelectListItem> ldidList = _db.TrafficHits
                                          .GroupBy(t => t.Id)
                                          .Select(g => g.First())
                                          .Select(c => new SelectListItem
                                          {
                                             Value = c.Id.ToString(),
                                             Text = c.ldid
                                         });