Devs can usually employ a duplicate app name in a different country _locale/country/region_ from an existing app name use, during submittal/review. Apple's meta data checks will not alert and app review won't automatically complain/react. An exception of course is blatant trademark/copyright poaching, etc.
If the submittal process doesn't auto push back, the only thing stopping you in that example is your intent, ethics etc., or later, a downstream complaint from whomever is also using that name elsewhere and their ability to get Apple's attention resulting in contacting you about said complaint.
How this plays out in the future is far too subjective to predict, but as always, a plan B is your friend. Speak up if you need hints on what types of plan B might prove useful.
And finally, if you're considering variations on 'demo', know that it is far better to apply the alphabet - 'Able Demo' will show up well before 'S Demo' or 'X Demo' etc., when users search, so don't go to all that trouble and then waste it with a shot to the foot 😉
Good luck.