I think the situation you mention is best described by the idiom "If you can't beat it, lead it". People who would be able to find a duplicate, or downvote a poor answer, are confronted with a crowd of new users coming from the HNQ link, who typically have 101 rep and for this reason cannot downvote or VTC, even if they agree. All they can do is upvote.
This kind of voting asymmetry leads to the only possible behaviour that has a visible impact and is rewarded with reputation - write your own answer, no matter how poor or controversial it is. Personally I came to the conclusion that unless this voting asymmetry is somehow fixed (by either forbidding 101 rep users to vote at all, or letting them downvote), any other measure will be akin to symptomatic treatment at best.
To clarify: I didn't mean that "If you can't beat it, lead it" should be everyone's modus operandi. I'm saying that someone who takes time and effort to search for duplicates and downvote bad answers is penalized with rep loss and resentful comments, whereas someone who adds yet another answer to the lot (good or otherwise) is rewarded. Bad answers won't go anywhere until that changes.