The proper way to have content in only one place is to mark the question as a duplicate. Though there are some exceptions to that rule (the ones you found being a notable exception). The odds are pretty good that the question is a duplicate. If that isn't the case though, then you may copy your own content (and should cite and attribute all other content)
Often times, this content may be excerpted from a larger work. For example, I wrote this 80,000 ish character answer (which really spans 3 answers due to a character limit on the answer field) and then excerpted some relevant parts here and here but in each case, I tailored my answer with some tweaking in the middle and added an introduction and conclusion.
I wouldn't want to simply link to the original answer because 1) what part did I intend for you to read? (it would be unclear) and 2) we prefer the original content to be included with the answer and it might even be deleted as a "link-only answer (and not spam)" which is one of available reasons for flagging (and deleting) and answer for moderators.
In terms of attribution, self-plagarism isn't much of an issue. It it is your own work, if you don't attribute it to yourself, then that's up to you. Generally, you might want to do that to advertise yourself, but at the end of the day, it only hurts you and if you have a good (or even bad) reason for failing to attribute yourself, that's your own decision to make.
So, in short, our policy on copying your own works: Copy away!