This may be a duplicate of Should we stop auto-deleting dupe-closed questions? but the wording of the staff answer seems to indicate that it was effectively declined in 2012. It's now 8 years later and I believe that this should be revisited.
The full set of rules for automatic deletion is found here. I could not find a more recent set of rules.
On Software Engineering, this question (10k on Software Engineering required to view) was posted on April 1. It was closed on April 1 by the asker accepting the duplicate suggestion. It was automatically deleted on April 11 by Community as part of the automatic clean-up process under the rule RemoveAbandonedClosed.
The assumption that I am making is that duplicates are good signposts. That is, they help people find the answers that they are looking for. This includes people searching via external search engines, people searching a single site, and the recommendations that are displayed when people are asking a question. As such, having duplicates are not necessarily bad. I would suggest that any duplicate that is otherwise on-topic, well written, appropriately tagged, and is not an exact copy-and-paste duplicate of another question is a good thing as it can help promote the site, especially through external traffic.
Based on the defined rules, my understanding is that this question may trigger several automatic deletion triggers. After 30 days, it would be deleted if has a negative score and has no answers. Even if the net score was not negative, it may have still been deleted after 365 days if it had a score of 0 (or less than 1 if the asker's account is deleted), has no answers, or has a low (<= age of question in days * 1.5) view count.
I believe that duplicates should be exempt from these rules.
The particular question that I observed as automatically deleted has 1 up-vote and 2 down-votes, resulting in a net score of -1. However, it's not clear why this question was down-voted. In my opinion, being a duplicate alone is not a good reason for down-voting, yet there may be people in the community who disagree. We have limited control over the individual voting habits of people who have a sufficient reputation score. I believe that voting may be a good indicator of the ability of the post to serve as a threshold, but a low negative score should be insufficient to trigger the automatic deletion. I'm not sure what a good threshold would be, but I would recommend looking at a score of -3 with at least 5 voters. To enhance this, perhaps even look at the time frame - check the question so many days after posting or after closure to see the voting score.
The view count also shouldn't trigger the automatic deletion for duplicates. The point is not necessarily to get eyes on the question but to be an index. A low view question still shows up in searching and could show up in recommendations.
The lack of answers and comments also has no bearing on duplicates. I'd even argue that a duplicate without an answer is an even better signpost as it collates the answers on the canonical version of the question. Questions closed as a duplicate should be exempted as a rule.
In short, questions that are closed as a duplicate would preferably be never automatically deleted. However, if automatic deletion is necessary, the threshold for deletion should be raised to a much greater net negative score and not consider views, comments, or answers.
As gnat pointed out in the comments, this question was identified as being removed by RemoveAbandonedClosed. However, the documentation for this cleanup reason excludes questions closed as duplicates - the earliest that it should have been deleted was with the 30-day RemoveDeadQuestions script since it was negatively scored, unanswered, and not locked. One upvote would have pushed it to the 365-day RemoveAbandonedQuestions script, but it would have likely be eligible for that because it did not have a positive score, was unanswered, was not locked, didn't have comments, and wasn't on a meta site.
has a score of 0 or less, or a score of 1 and a deleted owner has no answers is not locked has view count <= the age of the question in days times 1.5 has 1 or 0 comments isn't on a meta site