I've read differing opinions that submitting the same article isn't entirely bad. Google will ignore duplicate content and only show one of the articles.
Since some articles sites are indexed faster than other, and some article sites approve articles faster than others - it's all the same to me because I want my article out there as soon as possible.
I agree that posting slightly different content (20% is my magic number), changing up headlines, ordering of paragraphs etc can help too.
Even though Google may only index one article, the customer, who is human will be reading it - and if they find your article in a few places it might lose it's magic selling value a little.
- Rob