Article rewrite

I have a large number of articles {about 400 words each}, on a number of different topics. I want to make them unique again, as I suspect some people have copied them off my websites? Suddenly the search engines are saying I have duplicated content! Next time, I will take better care to prevent stealing! I simply want somebody to take the articles and rewrite each sentence, slightly changing the wording, but keeping the same or similar meaning if possible? That will make the article unique again? I find if I do this myself, it takes me about 15? minutes per article? I would like a price to do one article, or alternatively to give me some idea of the likely cost of perhaps doing them in batches of say ten articles, on the same topic? I have them as .txt files, suitable for Notepad?