Redirect url to same page

Question

I created pagination like example.com/shop/1 and example.com/shop/2,3,4,5... on shop.php i want to redirect the example.com/shop/1 to the example.com/shop only for no duplicate content issues. When i am using header redirect i am getting the header already sent error. Where i am mistaking?

<?php
require_once 'header.php';
$limit = 1;
$posts->post_type = 'products';
$currentPage = (isset($params['paged']) && is_numeric($params['paged']) ) ? $params['paged'] : 1;
$total_records = $posts->get_posts_total_record();
$pagination = new Pagination($currentPage, $total_records, $limit, BASE_URL.'/shop/', '', '');
$paginationStart = ($currentPage - 1) * $limit;
$rows = $posts->get_posts(
    array
    (
        "post_type"  => 'products',
        "start"  => $paginationStart,
        "limit"  => $limit,
        "order"  => 'id',
        "sort"  => 'DESC', // DESC / ASC
        //"offset"  => $limit
    )
);
$rowsCount = $rows->rowCount();
?>

<section class="inner-header">
    <div class="title">Our Products</div>
    <div class="breadcrumb"><a href="<?php echo BASE_URL; ?>">Home</a> / Our Products</div>
</section>

<section class="portfolio-page">
    <div class="border"></div>
    <div class="title">Products</div>
    <div class="boxs">
        <?php if($rowsCount > 0) { ?>
            <?php 
            foreach($rows as $row) { 
            $meta = $posts->get_meta(
                array
                (
                    "post_id"  => $row['id'],
                    "meta_key"  => "video_url"
                )
            );
            ?>
                <div class="box" data-url="<?php echo $meta['meta_value']; ?>" data-title="<?php echo $row['title']; ?>">
                    <?php echo $posts->get_thumbnail(array("id" => $row["id"],"featured" => true,"class" => 'featured',"width" => 370,"height" => 271)); ?>
                    <div class="content">
                        <div class="cont-title"><?php echo $row["title"]; ?></div>
                        <div class="cont-subtitle"><?php echo $row["excerpt"]; ?></div>
                    </div>
                </div>
            <?php } ?>
        <?php } else { ?> 
            <?php echo 'Not Found!'; ?>
        <?php } ?>
    </div>
    <?php if($rowsCount > 0) { ?>
        <div class="d-flex justify-content flex-column align-items">
            <?php echo '<div class="pagination-details">'.$pagination->getPaginationSummary().'</div>'; ?>
            <?php echo '<div class="pagination">'.$pagination->getPaginationHtml().'</div>'; ?>
        </div>
    <?php } ?>
    <div class="btns">
        <a href="/order-now" class="btn">Get Started</a>
    </div>
</section>

<?php
    require_once 'footer.php';
?>