diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
index c947609a35698..e952f2f22d88a 100644
--- a/src/wp-includes/block-supports/custom-css.php
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -25,6 +25,18 @@
  *     },
  *     ...
  * } $parsed_block
+ * @phpstan-return array{
+ *     blockName: string|null,
+ *     attrs: array{
+ *         className?: string,
+ *         style?: array{
+ *             css?: string,
+ *             ...
+ *         },
+ *         ...
+ *     },
+ *     ...
+ * }
  */
 function wp_render_custom_css_support_styles( $parsed_block ) {
 	$custom_css = $parsed_block['attrs']['style']['css'] ?? null;
@@ -49,20 +61,30 @@ function wp_render_custom_css_support_styles( $parsed_block ) {
 		? "$existing_class_name $class_name"
 		: $class_name;
 
-	_wp_array_set( $parsed_block, array( 'attrs', 'className' ), $updated_class_name );
+	$parsed_block['attrs']['className'] = $updated_class_name;
 
 	// Process the custom CSS using the same method as global styles.
 	$selector      = '.' . $class_name;
 	$processed_css = WP_Theme_JSON::process_blocks_custom_css( $custom_css, $selector );
 
 	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.
+		/**
+		 * Skip CSS that has already been added. Blocks with identical attributes
+		 * share the same class name and processed CSS via {@see wp_unique_id_from_values()},
+		 * so the same style would otherwise be enqueued more than once (e.g. inside
+		 * a Query Loop or when blocks share identical custom CSS).
 		 */
-		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;
diff --git a/tests/phpunit/tests/block-supports/wpRenderCustomCssSupportStyles.php b/tests/phpunit/tests/block-supports/wpRenderCustomCssSupportStyles.php
index 0048b379f8f33..e9556254311b8 100644
--- a/tests/phpunit/tests/block-supports/wpRenderCustomCssSupportStyles.php
+++ b/tests/phpunit/tests/block-supports/wpRenderCustomCssSupportStyles.php
@@ -14,6 +14,9 @@ class Tests_Block_Supports_WpRenderCustomCssSupportStyles extends WP_UnitTestCas
 	public function set_up() {
 		parent::set_up();
 		$this->test_block_name = null;
+
+		global $wp_styles;
+		$wp_styles = null;
 	}
 
 	public function tear_down() {
@@ -21,6 +24,10 @@ public function tear_down() {
 			unregister_block_type( $this->test_block_name );
 		}
 		$this->test_block_name = null;
+
+		global $wp_styles;
+		$wp_styles = null;
+
 		parent::tear_down();
 	}
 
@@ -264,4 +271,60 @@ public function data_does_not_add_class_name() {
 			),
 		);
 	}
+
+	/**
+	 * Tests that CSS is enqueued only once when the same block is rendered
+	 * multiple times, as happens inside a Query Loop.
+	 *
+	 * @ticket 65268
+	 *
+	 * @covers ::wp_render_custom_css_support_styles
+	 */
+	public function test_css_not_duplicated_on_repeated_renders(): void {
+		$this->test_block_name = 'test/custom-css-query-loop-dedup';
+		register_block_type(
+			$this->test_block_name,
+			array(
+				'api_version' => 3,
+				'attributes'  => array(
+					'style' => array(
+						'type' => 'object',
+					),
+				),
+				'supports'    => array( 'customCSS' => true ),
+			)
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-query-loop-dedup',
+			'attrs'     => array(
+				'style' => array(
+					'css' => 'font-size: 2em; /* query-loop-dedup-test */',
+				),
+			),
+		);
+
+		// Simulate the same block being rendered multiple times inside a Query Loop.
+		$result = wp_render_custom_css_support_styles( $parsed_block );
+		wp_render_custom_css_support_styles( $parsed_block );
+		wp_render_custom_css_support_styles( $parsed_block );
+
+		// Extract the generated class name from the first render's result.
+		$this->assertSame( 1, preg_match( '/(?:^|\s)(wp-custom-css-\S+)/', $result['attrs']['className'] ?? '', $matches ) );
+		$class_name = $matches[1];
+
+		// Count how many times the CSS selector for this block appears in the enqueued inline styles.
+		$inline_styles = (array) wp_styles()->get_data( 'wp-block-custom-css', 'after' );
+		$occurrences   = 0;
+		foreach ( $inline_styles as $style ) {
+			$this->assertIsString( $style );
+			$occurrences += substr_count( $style, '.' . $class_name );
+		}
+
+		$this->assertSame(
+			1,
+			$occurrences,
+			'CSS should be enqueued exactly once even when the same block renders multiple times.'
+		);
+	}
 }