Hypothetically, if that was the case it doesn't cover the closing of questions that are not duplicates.
I would contend that the "close as something that you have an answer on" is less driven by "I want more votes on the answer" but rather "I know where to find this answer."
Alternatively, if the person didn't close it as an answer you would instead have the person copying and pasting the same answer into the new question - which would accomplish the same thing (more votes for your answers) and further fragment the "one place to look" ideal.
From the perspective of the site and curation of information, a given answer should appear in one and only one question. Closing a question as a duplicate serves to further that goal. Copying and pasting answers ( https://meta.stackoverflow.com/questions/320351/how-to-handl... ) to questions that would be duplicates is frowned upon. Diamond mods get such behavior raised to them as a system flag ( https://meta.stackoverflow.com/a/317988/ ) - "Duplicate answers (auto) - raised on each duplicate answer"
For what it's worth, users with a gold badge are incentivized to close something as a duplicate if possible, rather than marking it as unclear, unfocused etc. even if those things are also true. This increases the chance that the OP gets some useful information anyway, and allows the curator to act unilaterally - avoiding the risk of someone trying to answer in the mean time.
In general, answering a question that you're actively curating is looked down upon on meta (it raises suspicion of vote fraud; and yes, moderators do care about that quite a bit, even if they recognize how broken the reputation system is) unless you've also asked the question intentionally as a canonical duplicate target (https://meta.stackoverflow.com/questions/426205) and you're writing a new answer from scratch. And proper citations are required for anything you get from someone else - whether it's another SO answer or something elsewhere on the Internet.
I would contend that the "close as something that you have an answer on" is less driven by "I want more votes on the answer" but rather "I know where to find this answer."
Alternatively, if the person didn't close it as an answer you would instead have the person copying and pasting the same answer into the new question - which would accomplish the same thing (more votes for your answers) and further fragment the "one place to look" ideal.
From the perspective of the site and curation of information, a given answer should appear in one and only one question. Closing a question as a duplicate serves to further that goal. Copying and pasting answers ( https://meta.stackoverflow.com/questions/320351/how-to-handl... ) to questions that would be duplicates is frowned upon. Diamond mods get such behavior raised to them as a system flag ( https://meta.stackoverflow.com/a/317988/ ) - "Duplicate answers (auto) - raised on each duplicate answer"