Sudoku Blanks - c++

I have to write a program that takes in a Sudoku square(with all slots filled) and randomly assigns 25 blanks to be filled in. This is what I have so far but because this code has the chance to generate the same position in the array more than once I'm getting a varying number of blanks(17-21). I'm wondering if there is a simple way to get it to output 25 blanks no matter what. My print function inserts a blank if the value is zero at any spot in the array.

void insertBlanks(int square[9][9])
{
    srand(time(NULL));
    int i = 0;
    while(i < 25)
    {
        int tempOne = rand() % 9;
        int tempTwo = rand() % 9;
        square[tempOne][tempTwo] = 0;
        i = i + 1;
    }
}