Permit merging contacts when both have a category

contactcats.php

@@ -55,3 +55,18 @@ function contactcats_civicrm_navigationMenu(&$menu) {
   ]);
   _contactcats_civix_navigationMenu($menu);
 }
+
+/**
+ * Implements hook_civicrm_merge()
+ *
+ * @link https://docs.civicrm.org/dev/en/latest/hooks/hook_civicrm_merge/
+ */
+function contactcats_civicrm_merge($type, &$data, $mainId = NULL, $otherId = NULL, $tables = NULL) {
+  if ($type === 'sqls' && $otherId) {
+    // delete the 'other' contact's category; we cannot have two per contacts.
+    \Civi\Api4\ContactCategory::delete(FALSE)
+      ->addWhere('id', '=', $otherId)
+      ->execute();
+    Civi::log()->debug("ContactCategory: deleted category for $otherId before merge into $mainId");
+  }
+}