Issue #3342527 Post update script does not check if slack_link_names is already set

slack.post_update.php

@@ -9,8 +9,9 @@
  * Add new configuration item slack_link_names.
  */
 function slack_post_update_slack_link_names() {
-  \Drupal::configFactory()
-    ->getEditable('slack.settings')
-    ->set('slack_link_names', TRUE)
-    ->save(TRUE);
+  $config = \Drupal::configFactory()->getEditable('slack.settings');
+
+  if ($config->get('slack_link_names') === NULL) {
+    $config->set('slack_link_names', TRUE)->save(TRUE);
+  }
 }