A Set is a list with only unique values. (No duplication.)
function storagePushToSet([key, value]) {
    log('storagePushToSet', key, value);

    const item = getLocalStorageItem(key);
    const list = Array.isArray(item) ? item : [];

    if (list.indexOf(value) === -1) {
      list.push(value);
    }

    setLocalStorageItem(key, list);
  }