@GH0STEDbyME @DrunkyMacGreed
Let me clarify a bit this issue as I'm used to it since 2017.
Hero Duplication is a glitch that can be abused. People exploit that glitch to spawn with copies of the same heroes. DICE had been able to fix one method to trigger it, but in the same patch they did this, they implemented the Squad System which led people to find another way to trigger it (which was related to the infinite spawn glitch that was never solved either). Since then the old method started working again, but I'm unsure if it still does, I haven't tested this myself in ages.
However, not only that, but if you spam the class you want to spawn with before you're able to spawn, there's a chance you'll trigger this glitch unintentionally, which explains why many times (especially in non Hero only modes) you see this happening with people that doesn't seem to know what's going on.
So if you see this in HvV, just leave this lobby and search for another one, because more times then not the enemy team is just abusing of a glitch to create an unfair match.
To be more specific: This glitch doesn't actually allow you, the glitcher, to spawn with a Hero that's already being played. Rather, you, the glitcher, picks a Hero without the game acknowledging that Hero was picked, and therefore still allowing other players to choose it. So the glitcher actually allows another player to pick the same Hero as he did, he can't choose a hero that has already been normally chosen by one of his allies. If the next player to pick the same hero glitches it as well, then the game will still allow for a 3rd player to get the same Hero. If at least 3 glitchers are on a team, they can allow all four members to get the same hero, so long the glitchers do it first, and the last teammate decides to choose the same hero as them because it'll be available in the selection screen so long all the glitchers glitched the Hero when selecting it.
To undo the glitch you must kill the glitchers (unless they are a full 4 man premade and all are glitching everytime they pick the character). So long one of the players using the duplicate hero didn't perform the glitch, if you kill the glitchers, they won't be able to pick that hero again until the legit player decides to change his hero. That doesn't prevent the glitchers from glitching another hero though.
I hope that explains this issue. And don't expect any fixes for this. This glitch has endured since launch, I doubt now that DICE abandoned the game that they'll manage to fix it.