Why People Use to Spam Classified Websites

I am wondering that why people post same ad again and again which is already published by some one other or by the same person. I am inspired by this nature of the peoples because I also own a classified website where some people post same ad again and again. So due to duplicate content in that website I have to forcefully delete that duplicate ad. So I really hate spamming and I think no one like spamming.

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Why People Use to Spam Classified Websites