i was wondering about paid articles

That will depend on what kind of rights you're giving up. If you give up all rights to an article then you cannot sell your article elsewhere. Usually if you sell your articles for an upfront payment, the buyer would want all rights to the article so you have no more rights over the article and you can't sell the article to anyone else or you can be sued for infringement. So read about the rights you're giving up.

You have to read the fine print on any assignments you accept from these sites. Some sites take ownership of your work, some don't let you republish, and some let you retain the rights. You should follow the guidelines for your specific article because it is illegal to go against these guidelines after you submit.

Who gets the rights depends on the site and whether they offer you alternatives. On Associated Content, you can sell them the rights or keep them. On Constant Content, you have several ways of selling your work, and the more rights you give the buyer, the more you can charge. Some sites demand all rights, some sites have time or other kinds of limitations. You need to check each site individually to find out exactly what you're selling.

As for duplication, yes, it's both ethical and legal to sell an article more than once, just as long as you know that some sites require first publication and don't try to sell them something that's already been published somewhere else.

in my experience, when i sold my articles to one particular website/group/person, i was not allowed to publish them on any other websites. they told me not to. maybe it depends on your agreement with them.