@@ -2656,6 +2656,16 @@ function img_caption_shortcode( $attr, $content = '' ) {
 		return $output;
 	}
 
+	/**
+	 * @var array{
+	 *     id: string,
+	 *     caption_id: string,
+	 *     align: string,
+	 *     width: string,
+	 *     caption: string,
+	 *     class: string,
+	 * } $atts
+	 */
 	$atts = shortcode_atts(
 		array(
 			'id'         => '',
@@ -2675,24 +2685,30 @@ function img_caption_shortcode( $attr, $content = '' ) {
 		return $content;
 	}
 
-	$id          = '';
-	$caption_id  = '';
-	$describedby = '';
+	$id               = '';
+	$caption_id       = '';
+	$describedby      = '';
+	$unique_id_value  = '';
+	$caption_id_value = '';
 
 	if ( $atts['id'] ) {
-		$atts['id'] = sanitize_html_class( $atts['id'] );
-		$id         = 'id="' . esc_attr( $atts['id'] ) . '" ';
+		$atts['id']      = sanitize_html_class( $atts['id'] );
+		$unique_id_value = (string) preg_replace( '/-1$/', '', wp_unique_prefixed_id( $atts['id'] . '-' ) );
+		$id              = 'id="' . esc_attr( $unique_id_value ) . '" ';
 	}
 
 	if ( $atts['caption_id'] ) {
+		// User explicitly provided a caption_id - make it unique.
 		$atts['caption_id'] = sanitize_html_class( $atts['caption_id'] );
-	} elseif ( $atts['id'] ) {
-		$atts['caption_id'] = 'caption-' . str_replace( '_', '-', $atts['id'] );
+		$caption_id_value   = preg_replace( '/-1$/', '', wp_unique_prefixed_id( $atts['caption_id'] . '-' ) );
+	} elseif ( $unique_id_value ) {
+		// Derive from the already-unique figure ID - guaranteed unique, no need for second call.
+		$caption_id_value = 'caption-' . str_replace( '_', '-', $unique_id_value );
 	}
 
-	if ( $atts['caption_id'] ) {
-		$caption_id  = 'id="' . esc_attr( $atts['caption_id'] ) . '" ';
-		$describedby = 'aria-describedby="' . esc_attr( $atts['caption_id'] ) . '" ';
+	if ( $caption_id_value ) {
+		$caption_id  = 'id="' . esc_attr( $caption_id_value ) . '" ';
+		$describedby = 'aria-describedby="' . esc_attr( $caption_id_value ) . '" ';
 	}
 
 	$class = trim( 'wp-caption ' . $atts['align'] . ' ' . $atts['class'] );
 
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.1-alpha-62697';
+$wp_version = '7.1-alpha-62698';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
Original file line number	Diff line number	Diff line change
`@@ -16,7 +16,7 @@`
`16`	`16`	`*`
`17`	`17`	`* @global string $wp_version`
`18`	`18`	`*/`
`19`		`-$wp_version = '7.1-alpha-62697';`
	`19`	`+$wp_version = '7.1-alpha-62698';`
`20`	`20`
`21`	`21`	`/**`
`22`	`22`	`* Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.`