Hello, I have a site based on a portal engine. When I started the site it was not optimized. Now I want to make some SEO but it will result that the content will be accessible in form mysite/?item=1 and mysite/articleTitle1. My problem is how Google and other search engines will handle this case as it seems to be a duplicate content. So maybe the result will be worse than it is now. However I don't want to remove the old format as I have a lot of link to the site with the old format. What do you think?
Yes it is SEO friendly, it's recommended that you use rewrites that combines URLs leading to the same places into one. Like how it's good to for example redirect http://yourdomain.com, http://yourdomain.com/index.php & http://www.yourdomain.com/index.php so that they all point to http://www.yourdomain.com
If 3 URLs redirect to 1 then google will index it 3 times or only once? Besides this google doesn't like doorway pages. The 301 code helps to avoid these problems?
a 301 is a permanent redirect it is basically you telling google "this page is now permanently at this new location so look here instead and update your index."