diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
index d4331ae3706ae..3df629afa72cf 100644
--- a/src/wp-includes/block-supports/custom-css.php
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -57,12 +57,21 @@ function wp_render_custom_css_support_styles( $parsed_block ) {
 
 	if ( ! empty( $processed_css ) ) {
 		/*
-		 * Register and add inline style for block custom CSS.
-		 * The style depends on global-styles to ensure custom CSS loads after
-		 * and can override global styles.
+		 * Track which class names have already had their CSS enqueued to prevent
+		 * duplicate styles when the same block is rendered multiple times inside
+		 * a Query Loop (render_block_data fires once per loop iteration).
 		 */
-		wp_register_style( 'wp-block-custom-css', false, array( 'global-styles' ) );
-		wp_add_inline_style( 'wp-block-custom-css', $processed_css );
+		$handle = 'wp-block-custom-css';
+		if ( ! wp_style_is( $handle, 'registered' ) ) {
+			wp_register_style( $handle, false, array( 'global-styles' ) );
+		}
+		$after_styles = wp_styles()->get_data( $handle, 'after' );
+		if ( ! is_array( $after_styles ) ) {
+			$after_styles = array();
+		}
+		if ( ! in_array( $processed_css, $after_styles, true ) ) {
+			wp_add_inline_style( $handle, $processed_css );
+		}
 	}
 
 	return $parsed_block;