People have turned mirrored sites into a duplicate content money machine. All of the spinning and fear of duplicate content is nothing but a way for marketers to make bucks on the lack of knowledge of the people who read their trash.
Now, as far as articles, this post should help the people who think that ezinearticles or any solo directory is the answer.
Article Marketing and SEO. The Super Advantages of Articles
By limiting yourself to just a few directories, you are robbing yourself of a lot of traffic and natural search results.
The article directories like ezine articles does not want you to submit your articles to other directories, because each article has the potential to bring them traffic and ad revenue, and if they are the only one who has that content, they have a monopoly on that content with no comp. Sure they want you to submit it to them only.
People need to stop and think of the motives before assuming that these places are telling you the truth.
Larwee already gave a link about duplicate content, now you have the truth about multiple submissions of the same article, so now I will tell you about submission software. there are few that do the complete submission, most are just field loaders, so you get a chance to check it visualy before submission on each site, so it is no different than a hand submit, it just saves the time of copying and pasting the info in everytime or typing it in. So that assumption that it is better by hand is also false, 100% false.
Submission services: Do you think submission services just spam directories and continue to get away with it? Do you not think that their IP's could be blocked? The directories give them access to submit the articles and actually ask for the articles from the distribution service. So there is nothing wrong with a submission service.
Most directories require at least 350 words, and more are leaning to between 400 and 500 words. If it takes 700 words to get the info to the reader, then so be it, it does not have to be broken into 2 articles and it is not a waste. I have put out 1800 word articles with great effect.
As far as keyword density, that is not an issue anymore due to LSI, that is latent symantic indexing, and density issues were out in the cold from day one once this was implimented.
having one or more of your keywords in the titile and or content is a good idea since google can not reason. So relevence is based on the words, or similar words. but they do not have to be exact because of LSI. Example: If you have Aqua in the title of the article and are targeting water filters, google will credit it as relevent.
You can even write off topic articles and still get relevence credit with google. Here is an example: Lets say you have a site about fire safety, you could write an article on "how to put the fire back in your marriage" and you would get relevence credited to your article because it has at least one of your keywords in the title or content.
maybe this will clear some things up for you.