How much of an article should be changed in order for Google to not view it as duplicate content if submitted to other article sites? Any idea?
Write one article, then re-write it two more times keeping the same paragraph structure for a total of three unique articles. Then mix and match a random paragraph from each... that's good enough for now. Take a look at the free tool at http://www.thekaizenbusiness.com/hermes/ which will do this for you automatically.
I don't bother about submitting to anything more than ezine articles. I don't get enough traffic from the others to make it worth while.
Spend the money for 1 amazing article and dont stop advertising that article until you reach 10,000 backlinks or more. Do your research.
Dup content can happen but it takes a lot of trying to make that happen. I submit my articles to only about 10 different article sites but they are the top 10 sites. To be on the safe side I would change the title slightly every 5 submits or so. However all of my articles have always shown up in the top 20 on google after writing them and typically ezinerarticles is the best.