Prevent repetition of output characters in c

Here I have a code that is to output the vowels used in a string and ignore the consonants. However it repeats the output of the vowel if that vowel is used more than once in the input string. Is there some way that I can prevent repetition of characters outputted or some function that can achieve this same task?

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

int main() 
 {
  int i = 0, j = 0, k;
  char string[256], result[256];
  char vowel[10] = {'a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'};

  printf("Enter string:");
  fgets(string, 256, stdin);
  string[strlen(string) - 1] = '\0';

  while (string[i] != '\0') 
   {
    for (k = 0; k < 10; k++) 
     {
      if (vowel[k] == string[i]) 
       {
        result[j++] = string[i];
        break;
       }
     }
    i++;
   }

  result[j] = '\0';
  strcpy(string, result);

  printf ("\n");
  printf("Vowel used are: %s\n\n", string);

  return 0;

}