Unltracolumnizer

/* The app reads text from stdin -- the text is two columns of words
 * separated by tab characters -- removes the first column and the tab
 * characters, splits the remaining text into single letters, removes
 * duplicates and capitalizez the letters. The result is written to
 * stdout.*/

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

int main()
{
     char *input_buffer;
     (void)scanf("%m[a-z\t\n]", &input_buffer);
     char *output_buffer;
     size_t input_buffer_size = strlen(input_buffer);
     output_buffer = malloc(input_buffer_size);
     int input_buffer_index = 0;
     int copy_flag = 0;
     int output_buffer_index = 0;
     while (input_buffer[input_buffer_index] != '\0') {
          if (input_buffer[input_buffer_index] == '\t') copy_flag = 1;
          if (input_buffer[input_buffer_index] == '\n') copy_flag = 0;
          if (copy_flag != 0 && input_buffer[input_buffer_index] != '\t')
          {
               output_buffer[output_buffer_index] = input_buffer[input_buffer_index];
               output_buffer_index++;
          }
          input_buffer_index++;
     }
     output_buffer[output_buffer_index] = '\0';

     size_t output_buffer_size = strlen(output_buffer);

     size_t i, j;
     for (i = 0; i < output_buffer_size; ++i)
     {
          for (j = i + 1; j < output_buffer_size; ++j)
          {
               if (output_buffer[j] == output_buffer[i]) output_buffer[j] = '\0';
          }
     }

     for (i = output_buffer_size; i > 0; --i)
     {
          if (output_buffer[i - 1] != '\0') printf("%c\n", toupper(output_buffer[i - 1]));
     }

     free(output_buffer);
     return(0);
}