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Web hosting companies suspend hosting if someone uses duplicate content?

Does anyone know how youtube checks if some1 else uploaded the same video on youtube? What does youtube look for? Creation date? File size? what things?

I am starting to market online. Everyone says to post your articles to as many article directories as possible. But, don’t the search engines penalize you for duplicate content! I just wanted to know before I post my articles in all those directories!
Thanks

A suposed ‘SEO expert’ told me that because my website was accessable through both http://www.mysite.com as well as www.mysite.com that it would count as duplpicate content and be penalised by SE’s. I think this doesnt make sense.
What do you think?

i plan on deleting the content, and just putting it up in a blog format to allow for fresh content. however, might this cause a duplicate content penalty, since the search engine might think I’ve simply copied my content again? i obviously plan on adding fresh content as well.

Aren’t RSS feeds considered duplicate content? Also, if you post the same article in a hundred different article directories, that’s also considered duplicate content.

What are your thoughts on article spinning? Is Google developing a program that penalizes article spinning? Essentially, you can spin one article to make a hundred "different" articles to post to hundred different article directories. Is this legit? Can Google penalize you for this?

I run blog. MY friend gives some article to my site.
I like to find these article has it is duplicate content or not?
And I also check my site is it contain any duplicate content.How to perform this.