Translation system is completely broken / cannot save translation

I tried to develop some D8 code, but if I create a translation of a node it completely crashes. I cannot create translations at all.

I have enabled Articles to be translatable. The form comes up, but if you hit save you get this:

Drupal\Core\Entity\EntityStorageException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '1-en' for key 1: INSERT INTO {node_field_revision} (nid, vid, langcode, default_langcode, title, uid, status, created, changed, promote, sticky) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10), (:db_insert_placeholder_11, :db_insert_placeholder_12, :db_insert_placeholder_13, :db_insert_placeholder_14, :db_insert_placeholder_15, :db_insert_placeholder_16, :db_insert_placeholder_17, :db_insert_placeholder_18, :db_insert_placeholder_19, :db_insert_placeholder_20, :db_insert_placeholder_21); Array ( [:db_insert_placeholder_0] => 1 [:db_insert_placeholder_1] => 1 [:db_insert_placeholder_2] => en [:db_insert_placeholder_3] => 1 [:db_insert_placeholder_4] => English article [:db_insert_placeholder_5] => 1 [:db_insert_placeholder_6] => 1 [:db_insert_placeholder_7] => 1384106323 [:db_insert_placeholder_8] => 1384107075 [:db_insert_placeholder_9] => 1 [:db_insert_placeholder_10] => 1 [:db_insert_placeholder_11] => 1 [:db_insert_placeholder_12] => 2 [:db_insert_placeholder_13] => de [:db_insert_placeholder_14] => 0 [:db_insert_placeholder_15] => German article [:db_insert_placeholder_16] => 1 [:db_insert_placeholder_17] => 1 [:db_insert_placeholder_18] => 1384106323 [:db_insert_placeholder_19] => 1384107075 [:db_insert_placeholder_20] => 1 [:db_insert_placeholder_21] => 1 ) in Drupal\Core\Entity\FieldableDatabaseStorageController->save() (Line 693 of core\lib\Drupal\Core\Entity\FieldableDatabaseStorageController.php).