C - Generate M sets of N unique random numbers each

What we know:

• I have to generate M sets of N random numbers, each number being unique with respect to the other N-1 numbers in the same set. With respect to the other M-1 sets, the numbers can repeat sometimes.

• Every number must take a value between 1 and 90.

• I am using the rand() function of stdlib.h and seeding it with srand(time(NULL)).

Problem:

M usually takes a value between 100 and 1000 and with the current implementation, the algorithm only generates 2 (sometimes even 1) sets of random numbers, each one repeating 50% of times. So if M = 100 we will have SET1 SET2 SET1 SET2 ... SET1 SET2, with SET1 50 times and SET2 50 times, instead having the desired output: SET1 SET2 SET3 ... SET99 SET100.

What I've tried:

So far, I've used a frequency array (array that retains the number of times a number appears in a data structure) that holds how many times any number from 1 to 90 have been generated in 1 set. If it has already been generated, it tries to generate another one, and so on. This is the function:

int randomNumber(int frequency[]) {
    int x = rand()%90 + 1;

    if(frecquency[x] != 0)
        while(frequency[x] != 0)
            x = rand()%90 + 1;

    return x;
}

This function works as intended, it is able to generate N unique random numbers between 1 and 90. It just generates the same numbers always!