ВТорая попытка

#include <conio.h>
#include <stdio.h>
#include <locale.h>
#include <windows.h>

struct list
{
   list(char _elem = 0, list *_next = NULL) : elem(_elem), next(_next) {}

   char elem;
   list *next;
};

void print(list *l);
void unique(list *l);
char input(const char *path, list *l);

int main()
{
   setlocale(0, "");

   list *S = new list();

   input("symbols.txt", S);
   //unique(S);
   print(S);

   return 0 * _getch();
}

char input(const char *path, list *l)
{
   FILE *stream = NULL;
   fopen_s(&stream, path, "r");

   if (stream)
   {
      char elem = 0;

      list *d = new list();
      // Считываем все символы из файла.
      for (list *p = l; fscanf_s(stream, "%c", &elem, 1) != EOF && p; )
      {
         bool isUniq = true;
         for (list *q = d; q && isUniq; q = q->next)
            if (elem == q->elem) isUniq = false;

         if (!isUniq)
         {
            for (list **z = &l; *z; z = &((*z)->next))
            {
               if ((*z)->elem == elem)
               {
                  list *t = (*z)->next;
                  delete *z;

                  *z = t;
               }
            }
         }
         else
         {
            p->elem = elem;
            p->next = new list();
            p = p->next;

            list *ld = d;
            for ( ; ld->next; ld = ld->next);
            ld->elem = elem;
            ld->next = new list();
         }
      }

      fclose(stream);
      return 0;
   }
   else
   {
      perror("Завершение работы программы");
      return -1;
   }
}

void unique(list *l)
{
   for (list *q = l; q && q->next; q = q->next)
   {
      int dup_c = 0;

      for (list *t = q->next; t->next; )
      {
         if (t->next->elem == q->next->elem)
         {
            list *w = t->next->next;
            delete t->next;
            t->next = w;

            dup_c++;

            int a = 1;
         }
         else t = t->next;
      }

      if (dup_c)
      {
         list *w = q->next->next;
         delete q->next;

         q->next = w;

         int b = 2;
      }
      int c = 3;
   }
}

void print(list *l)
{
   printf_s("Результат работы программы: \n");
   for (list *p = l; p; p = p->next)
      printf_s("%c", p->elem);
}