function warnOnce(key: string, message: string) {
  if (process.env.NODE_ENV !== 'production') {
    if (warnedKeys[key]) {
      return;
    }

    console.warn(message);

    warnedKeys[key] = true;
  }
}
A simple function that prints a warning message once per session.

@param {string} key - The key used to ensure the message is printed once.
                      This should be unique to the callsite.
@param {string} message - The message to print