They had little choice: or else, they would have to synchronise the local elections electoral calendar across Europe, and force countries/states already allowing votes in multiple local elections nationally/regionally to remove that right.

Meanwhile, the French system is still not clear to me: how exactly does it prevent registration for multiple local elections? Is there a field on the registration form where you have to declare that you won't vote elsewhere (similar to those discussed for EP elections)? And/or is there communication between local election authorities?

_{*Lunatic*, n.
One whose delusions are out of fashion.}