What could be the reason for this error on `bin/magento setup:upgrade` after updating from 2.2.8 to 2.3.1?

> 
> SQLSTATE[42000]: Syntax error or access violation: 1061 Duplicate key
>  name 'WISHLIST\_CUSTOMER\_ID', query was: ALTER TABLE `wishlist` ADD
>  CONSTRAINT `WISHLIST_CUSTOMER_ID` UNIQUE KEY (`customer_id`)
> 
> 
> 

The `db_schema.xml` of the wishlist module contains this definition:

```
        <constraint xsi:type="unique" referenceId="WISHLIST_CUSTOMER_ID">
            <column name="customer_id"/>
        </constraint>

```

But it should not try to create the constraint if it is already there?

`SHOW CREATE TABLE whishlist` shows me:

```
CREATE TABLE `wishlist` (
  `wishlist_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'Wishlist ID',
  `customer_id` int(10) unsigned NOT NULL DEFAULT '0' COMMENT 'Customer ID',
  `shared` smallint(5) unsigned NOT NULL DEFAULT '0' COMMENT 'Sharing flag (0 or 1)',
  `sharing_code` varchar(32) DEFAULT NULL COMMENT 'Sharing encrypted code',
  `updated_at` timestamp NULL DEFAULT NULL COMMENT 'Last updated date',
  `name` varchar(255) DEFAULT NULL COMMENT 'Wish List Name',
  `visibility` smallint(6) DEFAULT '0' COMMENT 'Wish List visibility type',
  PRIMARY KEY (`wishlist_id`),
  KEY `WISHLIST_SHARED` (`shared`),
  KEY `WISHLIST_CUSTOMER_ID` (`customer_id`),
  CONSTRAINT `WISHLIST_CUSTOMER_ID_CUSTOMER_ENTITY_ENTITY_ID` FOREIGN KEY (`customer_id`) REFERENCES `customer_entity` (`entity_id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=34412 DEFAULT CHARSET=utf8 COMMENT='Wishlist main Table'

```

The entry in the `setup_module` table:

```
+------------------+----------------+--------------+
| module           | schema_version | data_version |
+------------------+----------------+--------------+
| Magento_Wishlist | 2.0.2          | 2.0.2        |
+------------------+----------------+--------------+

```