diff --git a/src/wp-admin/custom-header.php b/src/wp-admin/custom-header.php
index a06756a280..8b2973fc11 100644
--- a/src/wp-admin/custom-header.php
+++ b/src/wp-admin/custom-header.php
@@ -1164,7 +1164,8 @@ wp_nonce_field( 'custom-header-options', '_wpnonce-custom-header-options' ); ?>
 			'post_title' => basename($cropped),
 			'post_mime_type' => $image_type,
 			'guid' => $url,
-			'context' => 'custom-header'
+			'context' => 'custom-header',
+			'post_parent' => $parent_attachment_id,
 		);
 
 		return $object;
@@ -1180,8 +1181,12 @@ wp_nonce_field( 'custom-header-options', '_wpnonce-custom-header-options' ); ?>
 	 * @return int Attachment ID.
 	 */
 	final public function insert_attachment( $object, $cropped ) {
+		$parent_id = isset( $object['post_parent'] ) ? $object['post_parent'] : null;
+		unset( $object['post_parent'] );
+
 		$attachment_id = wp_insert_attachment( $object, $cropped );
 		$metadata = wp_generate_attachment_metadata( $attachment_id, $cropped );
+
 		/**
 		 * Filters the header image attachment metadata.
 		 *
@@ -1193,6 +1198,11 @@ wp_nonce_field( 'custom-header-options', '_wpnonce-custom-header-options' ); ?>
 		 */
 		$metadata = apply_filters( 'wp_header_image_attachment_metadata', $metadata );
 		wp_update_attachment_metadata( $attachment_id, $metadata );
+
+		if ( $parent_id ) {
+			$meta = add_post_meta( $attachment_id, '_wp_attachment_parent', $parent_id, true );
+		}
+
 		return $attachment_id;
 	}
 
@@ -1241,7 +1251,13 @@ wp_nonce_field( 'custom-header-options', '_wpnonce-custom-header-options' ); ?>
 
 		$object = $this->create_attachment_object( $cropped, $attachment_id );
 
-		unset( $object['ID'] );
+		$previous = $this->get_previous_crop( $object );
+
+		if ( $previous ) {
+			$object['ID'] = $previous;
+		} else {
+			unset( $object['ID'] );
+		}
 
 		$new_attachment_id = $this->insert_attachment( $object, $cropped );
 
@@ -1396,4 +1412,32 @@ wp_nonce_field( 'custom-header-options', '_wpnonce-custom-header-options' ); ?>
 
 		return $header_images;
 	}
+
+	/**
+	 * Get the ID of a previous crop from the same base image.
+	 *
+	 * @since 4.9.0
+	 *
+	 * @param  array $object A crop attachment object.
+	 * @return int|false An attachment ID if one exists. False if none.
+	 */
+	public function get_previous_crop( $object ) {
+		$header_images = $this->get_uploaded_header_images();
+
+		// Bail early if there are no header images.
+		if ( empty( $header_images ) ) {
+			return false;
+		}
+
+		$previous = false;
+
+		foreach ( $header_images as $image ) {
+			if ( $image['attachment_parent'] === $object['post_parent'] ) {
+				$previous = $image['attachment_id'];
+				break;
+			}
+		}
+
+		return $previous;
+	}
 }
diff --git a/src/wp-includes/js/customize-models.js b/src/wp-includes/js/customize-models.js
index 372cad6273..e2a5dfbcd3 100644
--- a/src/wp-includes/js/customize-models.js
+++ b/src/wp-includes/js/customize-models.js
@@ -164,6 +164,7 @@
 
 			this.on('control:setImage', this.setImage, this);
 			this.on('control:removeImage', this.removeImage, this);
+			this.on('add', this.maybeRemoveOldCrop, this);
 			this.on('add', this.maybeAddRandomChoice, this);
 
 			_.each(this.data, function(elt, index) {
@@ -187,6 +188,25 @@
 			}
 		},
 
+		maybeRemoveOldCrop: function( model ) {
+			var newID = model.get( 'header' ).attachment_id || false,
+			 	oldCrop;
+
+			// Bail early if we don't have a new attachment ID.
+			if ( ! newID ) {
+				return;
+			}
+
+			oldCrop = this.find( function( item ) {
+				return ( item.cid !== model.cid && item.get( 'header' ).attachment_id === newID );
+			} );
+
+			// If we found an old crop, remove it from the collection.
+			if ( oldCrop ) {
+				this.remove( oldCrop );
+			}
+		},
+
 		maybeAddRandomChoice: function() {
 			if (this.size() === 1) {
 				this.addRandomChoice();
diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index 6909d0fe2e..2ee66fbddf 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -1214,6 +1214,7 @@ function get_uploaded_header_images() {
 		$header_images[$header_index]['url'] =  $url;
 		$header_images[$header_index]['thumbnail_url'] = $url;
 		$header_images[$header_index]['alt_text'] = get_post_meta( $header->ID, '_wp_attachment_image_alt', true );
+		$header_images[$header_index]['attachment_parent'] = (int) get_post_meta( $header->ID, '_wp_attachment_parent', true );
 
 		if ( isset( $header_data['width'] ) )
 			$header_images[$header_index]['width'] = $header_data['width'];
diff --git a/tests/phpunit/tests/image/header.php b/tests/phpunit/tests/image/header.php
index a76ee2cd6e..a6467c49a6 100644
--- a/tests/phpunit/tests/image/header.php
+++ b/tests/phpunit/tests/image/header.php
@@ -137,4 +137,37 @@ class Tests_Image_Header extends WP_UnitTestCase {
 		$this->assertGreaterThan( 0, $cropped_id );
 	}
 
+	/**
+	 * @ticket 21819
+	 */
+	function test_check_get_previous_crop() {
+		$id = wp_insert_attachment( array(
+			'post_status' => 'publish',
+			'post_title' => 'foo.png',
+			'post_type' => 'post',
+			'guid' => 'http://localhost/foo.png'
+		) );
+
+		// Create inital crop object.
+		$cropped_1 = 'foo-cropped-1.png';
+		$object = $this->custom_image_header->create_attachment_object( $cropped_1, $id );
+
+		// Ensure no previous crop exists.
+		$previous = $this->custom_image_header->get_previous_crop( $object );
+		$this->assertFalse( $previous );
+
+		// Create the inital crop attachment and set it as the header.
+		$cropped_1_id = $this->custom_image_header->insert_attachment( $object, $cropped_1 );
+		$key = '_wp_attachment_custom_header_last_used_' . get_stylesheet();
+		update_post_meta( $cropped_1_id, $key, time() );
+		update_post_meta( $cropped_1_id, '_wp_attachment_is_custom_header', get_stylesheet() );
+
+		// Create second crop.
+		$cropped_2 = 'foo-cropped-2.png';
+		$object = $this->custom_image_header->create_attachment_object( $cropped_2, $id );
+
+		// Test that a previous crop is found.
+		$previous = $this->custom_image_header->get_previous_crop( $object );
+		$this->assertSame( $previous, $cropped_1_id );
+	}
 }