Why was my question Installing python: who is deadsnakes and why should I trust them? closed as a duplicate of Are PPAs safe to add to my system and what are some "red flags" to watch out for? even though I have linked to it when asking and explained why it doesn't answer my question? Was it really possible to trivially infer from there everything that was said in the great answer I received?
The reasoning here is that from the generic information about green and red flags for PPAs, you should be able to extrapolate this information and apply it to your specific case.
For instance, the PPA
deadsnakes has a Github page, which is updated daily.
All this points to a legitimate and good supported PPA.
Imagine if the same question is asked for each and every PPA in existence - this would give hundreds, maybe even thousands of questions with basically the same answer: Engage in your own thorough due diligence.
I would guess that the reason your question was closed as a duplicate was that either it was the same as the duplicated question or else the people judging it did not read either your question, the duplicated question, or both. The reason may just be similar titles, I see a lot of questions judged as duplicate just because they have similar titles.