message: Warn if someone tries to register 2 Custom Elements with the same tag.
  issue
diff: @@ -98,8 +98,15 @@ export function WebComponent(options: WebComponentOptions): (target: any) => any
  };
  }
  -// FIXME check for double registration with the same name.
  - window.customElements.define(options.tag.toLowerCase(), constructor);
  + // Check for double registration with the same tag.
  + const tag = options.tag.toLowerCase();
  + const previousRegistration = window.customElements.get(tag);
  + if (previousRegistration !== undefined) {
  + console.warn(`A Custom Element with name '${tag}' is already registered.`);
  + return constructor;
  + }
  +
  + window.customElements.define(tag, constructor);
  return constructor;
  };
  }
  