It is not acceptable to post the same article on multiple sites as you are penalized in the search engines for the same article being on multiple sites. While it provides more exposure for your writing it can have a negative effect.
There are two definitons of what is a duplicate article. The first is an exact copy and the other is one that changes some of the paragraph information but not enough to consider it to be a different article. From what I have learned you can take an article and restructure change the wording in at least 20-30%.making it appear to be a different article and not penalized by the search engines.