Should I find out (perhaps through the help of some other user) that my question or something very similar has already been asked, what should I do? Should I delete it? Should I post a pointer to its "twin"?
It is not clear to me from your question if you are aware that questions can be closed/marked as duplicates, which automatically creates a link.
You should be able to flag questions, including your own, as a duplicate:
Chose "flag" below the post and select "a duplicate" and specify the question of which it is a duplicate.
Then as it is your own question you further should even be able to confirm the duplicate right away; at least I believe this to be the case, but even if not it should be then marked as duplicate down the road by other users.
If you are worried there could be confusion, or that above does not work out for you, you can leave a comment pointing to the duplicate. (Rather do not edit the post to include this information; a link in the post will be added to the post automatically down the road.)
That being done, you can let the question sit for a while to wait if somebody opines it's not a duplicate or still wants to say something. This addresses the concern mentioned in a post given in a comment. I agree that spontaneous self-deletion can sometimes be slightly irritating for others engaging with the post.
After that consider if you believe the question adds something beyond the duplicate. In particular, does it contain some different keywords that might lead to somebody finding it easier than the existing duplicate or is there any other relevant information in your thread?
If yes, leave it around. If no, delete it after a while (say, a day or two after closure), if you feel like it, or just leave it around. There is no harm in having a decent duplicate marked as such.