If you see a question that has been asked before, vote to close it as a duplicate. So it gets a link to the earlier question (choose a good one if there are several candidates).
Naturally, frequently asked questions (and frequently referred questions) are those that get the most links to them. So the FAQ list grows and gets sorted over time, automatically.
It can be accessed, sorted by backlink count, via the frequent tab. And this works also when it's filtered by a tag (such as frequent float questions).
So I don't see a benefit in adding another post for each case on the main site, claiming to create a frequently asked question. A new post would start with zero backlinks on the "frequent" list, way behind existing frequently linked questions.
I prefer
- closing more obvious duplicates to strengthen the main version by adding links, so it gets pushed up the "frequent" list
- polishing that main post (or make it with its answers more comprehensive)
A good strategy for closing and linking, when spotting a duplicate: click its main tag, click the "Frequent" tab, so you can see the frequently asked (referred) questions for that tag, that are sorted by relevance, so there's a good chance to see the one to link to.