I made a multi-author website, where volunteer writers publish articles, and in return a description about them is shown under their articles, now I need them to sign a contract, with some statements like their ownership of the article, their consent for publishing it on the website.
I wonder if I should add a clause about writing exclusively for the website so not to generate duplicate content on the web and to preserve web SEO. I fear that would be discouraging for them since they are volunteers.
I could alternatively edit the clause to publish the content first on the website, or add the meta canonical; then they can publish it on their blog for example or wherever they want. That way would search engines identify the website as the original source? What do webmasters do in this case to not damage their SEO?