Creating one of each term is quite enough, thanks

migration_helpers.module

@@ -90,6 +90,11 @@ function migration_helpers_move_terms(
         $new_term->save();
         $tid = $new_term->id();
         \Drupal::logger('mass')->log(LogLevel::INFO, "Term $term_name created with TID $tid in vocabulary $destination_vocabulary.");
+
+        // Because we did the query only once, before we entered this loop, we
+        // need to add our newly created terms to the term data array we use to
+        // check for existing terms… or we'll keep creating duplicates.
+        $term_data[$new_term->id()] = $new_term->label();
       }
       $tids[] = $tid;
     }