from the animes I've seen and reading this (first time reading a danganronpa manga too), all the danganronpa series are stand alones like the Gundam series.
Although each cast in each game/anime are completely different, Theyre not exactly stand alones as they are all shown to be connected together in one way or another.
The reason why Killer Killer has a completely different name is because its wasnt suppised to be revealed that the it was part of the dangan ronpa series until ch.9 (the chapter where the mangaka is killed in a similar fashion to junko).
If you want to know how the dangan ronpa series are related to each other, read below (spoilers obviously)
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In an essence Junko is a crazed maniac with a despair kink and decides to create a literal apocalypse because of despairs!!1!1.
Dr1 features the cast who were classmates of junko and were protected by the outside world until the killing began.
Dr2 features prominent terrorists who attended the same school as the dr1 cast and became said terrorist thanks to junko. Naegi, the dr1 protagonist, decides to put them into a brainwashing video game because he claims that deep down they are actually good people. (Which imo is extremely illogical given that this is the apocalypse but naegi is a fan-favorite so imma keep my mouth shut)
Dr3:future shows what people decided to do with Naegi (he was about to be arrested when the killing start again). Turns out the president of the future foundation (the company in charge of dealing with naegi and the sdr2 cast), has a huge hope kink and set everyone up.
Dr3:despair shows how junko created the apocalyose. Junko loves her anime.
i have already watched the animation dangnaronpa and this is my first time reading this manga and saw that naegi is also here..so killer killer and danganronpa are connected and is another season of danganronpa it is just that the title is different??
(forgive me for my english grammar and hope u understand it)