From the faq post How should duplicate questions be handled?
Should duplicates be deleted?
... Duplicates that are word-for-word copies or that are so poorly written that they are not useful may be deleted by users with sufficient privilege.
How can one find those? Are there many of them?
One situation this is likely to happen is when the source and target (stub and master) are asked by the same OP. If someone is interested, I wrote a query in SEDE to find stubs with no answers, so if they are confirmed to be copies or poorly written questions, they can be deleted.
Self-duplicates with no answers (list of stub Qs)
Merged questions are also returned from the query, which are a special case because the stub is a locked post and does not qualify for auto-deletion.
For completeness, here are two more queries relating to self-duplicate questions.
The first one is from stub questions with answers, while the target is unanswered. Possible actions here are to remove the duplicate status and delete the target, or reverse the duplicate direction (stub becomes target, target become stub):
Self-duplicates: stub has answer, target does not (list of stub Qs)
The second one is about self-duplicates with answers both in stub and target questions. These are candidates for a merge with the possibility of subsequently deleting the stub:
How can one find those?
Duplicates and plagiarism appear to be detected and reported in the SOBotics Chat, a room that is dedicated to moderating content on Stack Overflow with the use of bots.
Source code is available, as is the dump of Stack Exchange. Features not supported could be suggested or a better tool written if one is so inclined.
Are there many of them?
Not for long, usually; it's a matter of discovery and action. The tools don't require "word for word exact copies" but they do need someone to look at their output, a few tools do work without further prodding.
There are also other similar projects: SEBotics - A side expansion of the SOBotics project to the SE network.
Duplicates with many downvotes are often worth deletion. The downvotes might be because the question is not significantly different from its sibling duplicates, so any reasonable search would have found one of the others: lacks research effort. The downvotes might be because it is so poorly written that it does not provide a clear sign post to the canonical question: unclear, not useful.
As usual, read before casting a delete vote.
