[hackermailman]

Some great general advice for talks is given here if you're interested http://www.cs.cmu.edu/~kayvonf/misc/cleartalktips.pdf but doesn't answer any of your submissions questions. Each conference I would imagine has guidelines how to apply to be a speaker or if more than one talk is allowed, or if it must be previously unpresented/new, just email them and ask.

An example of a really good talk is https://youtu.be/5c0BvOlR5gs where Shriram Krishnamurthi explains creating computer science curriculum for highschool students. Notice he moves around the room (if this is possible and not a huge arena), is able to prevent people from hijacking the talk like the one audience guy who rants off topic about early arithmetic curriculum, yet still manages to give an acceptable answer to his question, and promises to hang around afterwards to discuss questions with audience members who have multiple questions as a way to keep the talk moving on topic. The slides are perfectly presented as well moving effortlessly instead of those talks where they pause for each slide.