WordPress duplicating posts from single loop

Unless I’ve mis-interpreted what you are trying to accomplish, you added the foreach loop and you don’t need it. Having it run while inside a while loop seems to be duplicating your intentions. Remove the foreach loop altogether, delete the first wp_reset_postdata();, and un-comment the 2nd one:

<div id="carouselTestimonial" class="carousel slide" data-bs-ride="carousel">
    <div class="carousel-inner">
        <?php 
        $cptQuery = new WP_Query($args = [  
            'post_type' => 'client-testimonial',
            'post_status' => 'publish',
            'posts_per_page' => -1
        ]);
        $active="active";

        while ( $cptQuery->have_posts() ) : $cptQuery->the_post(); 
            setup_postdata( $cptQuery );
            $cptID = get_the_ID();
            $addTestHomePage = get_field('homepage_testimonial_carousel');
            $testComanyName = get_field('client_testimonial_company_name');
            $testFirstName = get_field('client_testimonial_client_first_name');
            $testLastName = get_field('client_testimonial_client_last_name');
            $testTitle = get_field('client_testimonial_client_title');
            $testImage = get_field('client_testimonial_client_picture');
            if( $testImage ):
                $testImgurl = $testImage['url'];
                $testImgtitle = $testImage['title'];
                $testImgalt = $testImage['alt'];
                $testImgcaption = $testImage['caption'];
            endif;
            $testAltImage = get_field('client_testimonial_secondary_image');
            $testQuote = get_field('client_testimonial_quote');

            if ($addTestHomePage === "yes") : 
                ?>
                <div class="carousel-item <?php echo $active;?>">
                    <?php echo $cptID; ?>
                    <div class="row">
                        <div class="col-3 col-md-3 col-sm-3">
                            <div class="imagesDiv">
                                <?php if ($testImage) { ?>
                                    <img class="images" src="<?php echo $testImgurl; ?>" title="<?php echo $testImgtitle; ?>" alt="<?php echo $testImgalt; ?>">
                                <?php } else { ?>
                                    <img class="images" src="<?php echo get_home_url(); ?>/wp-content/uploads/2023/08/<?php echo $testAltImage; ?>.webp" title="Stock Image of a Profile Picture" alt="This is a stock photo">
                                <?php } ?>
                            </div>
                        </div>
                        <div class="col-9 col-md-3 col-sm-9">
                            <?php if ($testQuote) : ?>
                                <div class="largeQuote"></div>
                                <div class="testimonialQuote">
                                    <?php echo $testQuote; ?>
                                </div>
                                <div class="byline"><?php echo $testFirstName; ?> <?php echo $testLastName; ?>, <?php echo $testTitle; ?></div><br />
                                <div class="byline"><?php echo $testComanyName; ?></div>
                            <?php endif; ?>                                        
                        </div>
                    </div>
                </div>
                <?php
                $active="";
            endif; //homepage carousel check
        endwhile;
        wp_reset_postdata(); ?>
    </div>
</div>
<a class="carousel-control-prev" href="#carouselTestimonial" role="button" data-bs-slide="prev">
    <span class="carousel-control-prev-icon" aria-hidden="true">Prev</span>
    <span class="sr-only">Previous</span>
</a>
<a class="carousel-control-next" href="#carouselTestimonial" role="button" data-bs-slide="next">
    <span class="carousel-control-next-icon" aria-hidden="true">Next</span>
    <span class="sr-only">Next</span>
</a>