Is it okay to post the same article you wrote on multiple website? (guest posts on other blogs and on your own blog) ?

It depends on the site where you are pitching your article. Ensure they are okay with reproducing the exact content on their site. Google is harsh on duplicate content site, so some sites might be sceptical about it.

A single line at the beginning or at the end of the article mentioning the reproduction of the article should be sufficient. But do not forget to do so.

Anyway if your content is top quality it will eventually find its way to reach the audience thanks to social media.