I've been kind of neglecting wordpress installations on my websites and noticed many showing duplicate content for pages showing under author and tags, tags and single post, categories and single post.

Should this be a concern? Whats the best way of fixing this?

Thanks

Plural is always better than singular. When it comes to singular term search your plural domain will still show for that keyword. IE if you search for "item" a domain with "items.com" will rank just as well as "item.com" because it already contains the singular version of that keyword.

You can confirm the above by performing a few searches and observing the results.

There is nothing wrong with having multiple sites / blogs on the same C block IP address. However, if you're trying to use your blogs to link to your products to boost SEO scores then you might want to consider other link building techniques in addition. Building backlinks from sites on same IP is okay, but you'll have greater benefits getting links from sites hosted on other servers.