export default function messages(state = getMessages(10), action) {
  switch (action.type) {
    case SEND_MESSAGE:
      const { message, userId } = action.payload;
      return {
        ...something
          }
        }
      };
    case EDIT_MESSAGE:
      const { message, userId } = action.payload;
      return state;
    default:
      return state;
  }
}

==> Syntax error : message et user_id déjà déclarés

soluce possible :
case SEND_MESSAGE: {
      const { message, userId } = action.payload;
      return {
        ...something
          }
        }
      };
}