I would love to share with you of some sites that are free of charge that you can use to check for duplicate content. Hope you will find they are useful to you.
Copyscape
The most famous and being widely used by a lot of article marketers would be Copyscape
For free one, you can check for 10 posts. After that, you will need to buy the credit, it is 5 cent per credit, and everytime you use to check for a post, 5 cents will get deducted. The minimum amount you need to pay for buying credit would be $10, and you will be able to check for duplication for 200 posts.
Duplichecker
