Permutation of String letters: How to remove repeated permutations?

Here is a standard function to print the permutations of characters of a string:

void permute(char *a, int i, int n)
{
   int j;
   if (i == n)
     printf("%s\n", a);
   else
   {
        for (j = i; j < n; j++) //check till end of string
       {
          swap((a+i), (a+j));
          permute(a, i+1, n);
          swap((a+i), (a+j)); //backtrack
       }
   }
} 

void swap (char *x, char *y)
{
    char temp;
    temp = *x;
    *x = *y;
    *y = temp;
}

It works fine but there is a problem, it also prints some duplicate permutations, exapmle:

if string is "AAB"

the output is:

AAB
ABA
AAB
ABA
BAA
BAA

This is having 3 duplicate entries as well.

Can there be a way to prevent this to happen?

--

Thanks

Alok Kr.