Questions about the rules of different games are inherently different questions, because the rules are different. By definition, different questions can't be duplicates of each other.
The only wrinkle is that not every different edition of a game is actually a different game. An example of this is that most editions of Call of Cthulhu are different publishings of the main book, but the rules they contain describe are effectively the same game. By contrast, D&D editions are the opposite: each published edition contains different rules, and is therefore a different game that merely “takes over” the name from the previous game of that name.
So go ahead and ask your question about Dragonborn in D&D 5e! The question about Dragonborn in D&D 3.5e can't answer a question about a different game's rules, so there is no concern about duplicating it.
(Sometimes people like to add a note to the ends of their questions in these cases though, saying “This is similar to [link to question], but for a different edition.” That's sometimes useful in case someone finds one question via, say, a search engine, but they were looking for a question about the other game — the link to the similar question about a different game can help them get where they were trying to go.)
Pathfinder & D&D 3.5e: A special case
Pathfinder and D&D 3.5e share many rules, being directly related games by different companies. In many cases a question about one will have the same rules and answers as the same question about the other. We handle these on a case-by-case basis, for lack of a better handling system.
So for Pathfinder and D&D 3.5e, err on the side of asking the question. More experienced users will help sort out if it's a duplicate or not. Often it won't be a duplicate, but even if it is one, that's okay: you'll have your answer, and duplicates are useful here and don't reflect poorly on the asker.