As we all know that there is a program that checks your article if it passes anti plagiarism. I want to have this program since I am now accepting article writing job. Please share your free software here if you don't mind.

Copyscape could be what you want. It is a site and not software, but they probably keep their database up to date to avoid problems.

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How does that work? It has a database of everything people have written? Or does it just scan the Internet for similar articles?