How to Order a List with Duplicates Without Repeats

Question

I want to know how to sort it like this. My code was working for the last value. If I change the index to another value, it won't work.

List<int> list = new List<int>(); 
            list.Add(1);
            list.Add(2);
            list.Add(3);
            list.Add(4);
            list.Add(5);
            list.Add(6);
            list.Add(7);
            var duplicates = list.GroupBy(x => x).Where(group => group.Count() > 1).Select(group => group.Key).ToList();
            int listcount = list.Count();
            if (duplicates.Count > 0)
            {
                int firstDuplicate = duplicates.First();
                int index = list.IndexOf(firstDuplicate);
                int lastDuplicate = duplicates[duplicates.Count - 1]; 
                int lastindex = list.LastIndexOf(lastDuplicate);

                int nextValue = lastDuplicate + 1;
                for (int i = index + 1; i <= listcount; i++)
                {
                    list.Insert(i, nextValue);
                    nextValue++;
                }
                if (list.Count > listcount)
                {
                    int count = list.Count() - listcount;
                    list.RemoveRange(listcount, count);
                }
            }