I asked a Meta Stack Exchange question about this recently.
I'm not eager to delete dupes at all. Dupes are awesome and deleting them indiscriminately or just for being dupes would be terribad. To quote myself:
I totally agree that it's good to have differently phrased versions of the same question around, even better if they have different details that help to show how broadly applicable the target is, or good answers that might, for example, be easier to follow than a highly detailed top answer to a canonical post.
But I do want to delete stuff that is bad
The post in question was undeleted, but the person who undeleted it said
That question is indeed rubbish
For me, that's the main issue. I don't want our awesome site to have any rubbish on it clogging up search results, whether it's duplicate rubbish or not. It might be argued that the ability to delete duplicate crap is the main reason to allow users with >10k to delete questions at all:
closed questions with no upvotes and no upvoted answers will be automatically deleted, unless they are closed as duplicates. This reflects the undisputed fact that duplicate questions can be extremely useful as signposts. I think this is right and good, but I also think that since the system does nothing to distinguish good dupes from bad dupes, it must be my responsibility as a user with >10k to try to clean up the mess, by voting to delete really bad dupes along with other stuff that deserves to go.
You say "unless we have a very good reason", and terdon says a "special reason" but we are talking about a post that could have easily been closed as too broad or primarily opinion based instead, and whose target is closed too.
Isn't being a poor quality duplicate of a bad question a good or special enough reason? The site has thousands and thousands of dupes. A tiny handful of them get deleted by voters. That suggests that we (the delete-voters) are being pretty selective.
this comment argued
Deleting makes it more likely that the same identical question will be asked again and we'll have to go through this all over again.
But, as I wrote in my MSE post I don't want anyone to land on low quality posts on our site, even on the way to a better question. I want the site to be awesome and free from crap, as far as I can make it so.
The argument that we should just leave the junk here to prevent people posting more junk just seems horrible to me, and negates the whole purpose of having delete votes. If some people want to have a site full of junk, then, I guess we will have to agree to disagree.