simple C crossword how to avoid duplicate words

I am new here and also new with coding. How can i find a 'bug' in a simple crossword program, written in C, that could, generate (unwanted) duplicate words?

All tips and suggestions are appreciated

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

#define ROWS 10
#define COLUMNS 10

char puzzle[ROWS][COLUMNS];

char allWords[20][10] = {
    "GIRL", "BOY", "SHIP", "CAT", "FOG",
    "KITE", "BAG", "STAMP", "ZOOM", "JOY",
    "CAR", "BUS", "VAN", "BOAT", "BIKE",
    "TURBO", "SCHOOL", "COVID", "VIRUS", "STAR"
};

char fourWords[4][10];

char getRandomCharacter()
{
    int r = (rand() % 26) + 65;
    return (char)r;
}   

void getFourRandomWords() // This code can generate duplicate words -- try to fix it
{
    int i;

    for (i = 0; i < 4; i++) {
        strcpy(fourWords[i], allWords[rand() % 20]);
    }
}

void createBlankPuzzle()
{
    int i, j;

    for (i = 0; i < ROWS; i++) {
        for (j = 0; j < COLUMNS; j++) {
            puzzle[i][j] = '*';
        }
    }
}

void displayPuzzel()
{
    int i, j, rowNum = 0;
    char x = 'A';
   
    // First display column names
    printf("  ");
    for (i = 0; i < COLUMNS; i++) {
        printf("%c ", x + i);
    }
    printf("\n");

    for (i = 0; i < ROWS; i++) {
        printf("%d ", rowNum);
        rowNum++;
        for (j = 0; j < COLUMNS; j++) {
            printf("%c ", puzzle[i][j]);
        }
        printf("\n");
    }
}

void putHorizzontalWord(char word[10])
{
    int rRow, rCol, ok, i;

    do {
        rRow = rand() % 10;
        rCol = rand() % 10;

        ok = 1;
        if (rCol + strlen(word) < 10) {
            for (i = 0; i < strlen(word); i++) {
                if (puzzle[rRow][rCol + i] == '*' ||
                    puzzle[rRow][rCol + i] == word[i])
                {
                    puzzle[rRow][rCol + i] = word[i];
                } else {
                    ok = 0;
                }
            }
        } else {
            ok = 0;
        }
    } while (ok == 0);
}

void fillPuzzleWithWords()
{
    int i, orientation;
    getFourRandomWords();

    for (i = 0;i < 4;i++) {
        orientation = 0; //rand() % 3; // To generate a random number from 0, 1, & 2
        if (orientation == 0) {
            putHorizzontalWord(fourWords[i]);
        } else
        if(orientation == 1)
        {
            // put word vertical
        } else {
            // put word diagonal
        }
    }
} 

int main(void)
{
    srand(time(NULL));

    createBlankPuzzle();
    fillPuzzleWithWords();
    displayPuzzel();

    getchar();
    return 0;
}

I executed the code about 10 times and it did reproduced the same word three times. The thing is that I do not understand how the word are picked/chosen from that list hence I do not know how to interact with these lines.

In order to produce distinct words, you can simply iterate over the words selected so far to check if the newly drawn word is different from all the previous ones:

void getFourRandomWords(void) {
    // draw 4 words from the list, no duplicates
    int i, j;
    for (i = 0; i < 4;) {
        const char *w = allWords[rand() % 20];
        for (j = 0; j < i; j++) {
            if (strcmp(fourWords[i], w) == 0)
                break;
        }
        if (j == i) {
            // add the word if it compared different from all previous ones
            strcpy(fourWords[i], w);
            i++;
        }
    }
}

The above naive method may be very inefficient if choosing n from n, as a matter of fact it is not even guaranteed to finish.

Here is a different approach to cap the number of iterations:

void getFourRandomWords(void) {
    int draws[4];
    // draw 4 words from the list, no duplicates
    for (int i = 0; i < 4; i++) {
        int n = rand() % (20 - i);
        int j;
        for (j = 4 - i; j < 4 && n >= draws[j]; j++) {
            draws[j - 1] = draws[j];
            n++;
        }
        draws[j - 1] = n;
        strcpy(fourWords[i], allWords[n]);
    }
}

Here is yet another approach with a single loop and linear time and space complexity:

void getFourRandomWords(void) {
    // list of possible word numbers
    int draws[20] = {  0,  1,  2,  3,  4,  5,  6,  7,  8,  9,
                      10, 11, 12, 13, 14, 15, 16, 17, 18, 19 };
    // draw 4 words from the list, no duplicates
    for (int i = 0; i < 4; i++) {
        // draw a number from i to 19
        int n = i + rand() % (20 - i);
        // select the word from the list
        strcpy(fourWords[i], allWords[draws[n]]);
        // swap the word number and the i-th slot
        // this way the selected number is removed from the list
        draws[n] = draws[i];
    }
}