Hi, thank you for the info. Seems like something changed in Menu Manager. But this does not have any influence on functionality of the component, it is just a "typo" problem (displayed twice instead of once) in Menu Manager view.
Anyway, I have reported it to Joomla!, so I hope, there will be a solution for this soon, see:
https://github.com/joomla/joomla-cms/issues/12682
Thank you for the info.
Jan