Get unique post in parent category

you can easily find solutions for this by using google.

— edit with solution for duplicates —
to filter out your duplicates fill an array with the ids. then check if the post is in the array if not print it , otherwise skip it.
you said you have solution for getting the posts so I won’t be typing code for that

$do_not_duplicate = array(); // set before loop variable as array

while ( have_posts() ) : the_post();
$do_not_duplicate[] = $post->ID; // remember ID's in loop

//now loop through the array
if ( !in_array( $post->ID, $do_not_duplicate ) ) { // check IDs
// display posts ...