Never send articles to many venues at the same time without informing all of the editors you've pitched to. It's best not to submit proposals to numerous sites at the same time, although this is often necessary for timeliness. The reason why you should never do this is because each publication has its own unique audience that will respond differently to your message. By submitting the same article to several publications, you are limiting yourself to one source of traffic and publicity for your story. It's important to understand that while pitching stories to media outlets is common practice for journalists, doing so without permission from your sources can have negative effects on your career. For example, if you write an article for a publication that uses material from another source without attribution, then they have the right to refuse future coverage from you. Furthermore, their readers may view you as untrustworthy if you don't give credit where it is due. So in summary, pitching stories to media outlets is common practice for journalists but it isn't recommended because it limits your source pool and could affect your career.