Removing duplicate items from an array using C language

You did not initialize the variable j. So using the uninitialized variable invokes undefined behavior.

It is better to form the array without duplicates in one loop instead of splitting the loop into a loop and one more statement after the loop.

You can write a separate function for example the following way as it is shown in the demonstrative program.

#include <stdio.h>

size_t remove_copy( const int a[], int b[], size_t n )
{
    size_t j = 0;

    for ( size_t i = 0; i < n; i++ )
    {
        if ( i == 0 || a[i] != b[j-1] ) b[j++] = a[i];
    }

    return j;
}

int main(void) 
{
    int a[] = { 1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5 };
    int b[sizeof( a ) / sizeof( *a )];
    const size_t N = sizeof( a ) / sizeof( *a );

    for ( size_t i = 0; i < N; i++ ) printf( "%d ", a[i] );
    putchar( '\n' );

    size_t n = remove_copy( a, b, N );

    for ( size_t i = 0; i < n; i++ ) printf( "%d ", b[i] );
    putchar( '\n' );

    return 0;
}

The program output is

1 2 2 3 3 3 4 4 4 4 5 5 
1 2 3 4 5 

Your code and logic is correct. But you did a mistake by not intializing j = 0. Because of that you were getting some gibberish values. So, I corrected and executed it and got correct answer.

#include <stdio.h>

int main() {
    int arr[12] = { 1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5 };
    int temp[12];
    int i, j = 0, k, n = 12;

    for (i = 0; i < n - 1; i++) {
        if (arr[i] != arr[i + 1]) {
            temp[j] = arr[i];
            j++;
        }
    }

    temp[j] = arr[n - 1];

    for (k = 0; k <= j; k++) {
        printf("%d\n", temp[k]);
    }

    return 0;
}