Joaquin Phoenix is returning as the Joker, alongside Robert De Niro, in the new Joker: Folie à Deux, as revealed by Todd Phillips, the director of the 2019 Joker movie, through his Instagram account with a snapshot of the script, although, the release date has not been set yet.
Additionally, the script seemingly offers a hint of what's to come from the new film with its subtitle reading "Folie à Deux", which roughly translates to "madness for two" and is a mental disorder affecting two or more individuals, seemingly hinting at the possibility of multiple Jokers being seen in the film.
The first Joker movie was very successful that it has won 2 Academy Awards, known as the Oscars, after being nominated for 11, in addition to winning 3 British Academy Film Awards (BAFTA) after being nominated for also 11, the most notable part is that Joaquin Phoenix has won the Best Actor in both. It was also the first R Rated movie to have grossed over a billion dollars.
Joker Folie à Deux, Script revealed by the Director