Suppose I have written an original story for fun not profit and now wish to post it on the internet for the delectation of others for free.
Is it acceptable practice to post it on several unrelated websites or would there be copyright or other legal problems in doing so.
I just wonder what rights any one website has over your content.
I don't really need legal opinions on this only if it's generally a done thing or not.
You may publish on multiple sites if (and only if) you are licensing each non-exclusively and retain the rights to your content (or at least enough rights to license it again). For example, read the Stack Exchange ToS, which allows you to freely publish your content elsewhere:
You agree that any and all content [...] is perpetually and irrevocably licensed to Stack Overflow on a worldwide, royalty-free, non-exclusive basis pursuant to Creative Commons licensing terms (CC BY-SA 4.0).
Compare the similar wording in the ToS for Tapas, a site where authors can self-publish webnovels and webcomics. It says you grant them "an ongoing, non-exclusive, worldwide, royalty-free license". Tapas also has a "premium novels" section (more akin to traditional publishing), where some of the stories are released under exclusive contracts.
Note that "perpetually and irrevocably" means that you can't (legally) force the content to be removed from the site you licensed it to.
Because a lot of sites offer non-exclusive publishing, a significant number of authors do publish on multiple sites.
Posting your content on multiple sites is very common. Whether it's your own personal site, facebook, twitter, instagram, patreon, etc. I know web-comic artists that post on most of these as well as one or two web-comic specific sites.
The specifics will depend on the site's terms of service. But in general, if they don't pay you for your content, they can't expect to have exclusivity. So you can post it wherever it will be accepted.
It might also be worth noting that most websites with user-generated content do not want to be liable for that content. So they are very explicit about that it's your content, that you publish it, and that they are just distributing it.
One thing you should keep in mind, though, is that when you publish something online, it may mean that you can not sell exclusive rights to anyone in the future. For example, according to the ToS, when you post on stack-exchange, the content is released under a creative common license. Anyone can then copy and use it (with attribution - CC BY-SA). So if you publish a story here, you may not be able to sell it to a publisher later on.
