Welcome to the Q&A Forum

Using this as an example: If google showed duplicate content for http://www.domain.com/seo.html  and http://www.domain.com/seo then you would want a 301 .htaccess redirect so anyone accessing the http://www.domain.com/seo.htm version of the page is automatically sent to http://www.domain.com/seo.

You can use the following in your .htaccess file.

RewriteRule ^/seo.html$ http://www.domain.com/seo/? [R=301,NC,L]

The benefit to having no file extension (.html) on the end of your URLs is it allows you to change the underlying framework or your website or even the programming language without the need for adding redirects each time you change (provided the website structure remains the same)

No, the $ is part of the .htaccess rule, try that and let me know if it works for you.

I would strongly recommend using the rule above on a development version of your website and not directly on the live server.

Try creating a .htaccess file on the root of your website with the following code.

Options +FollowSymLinks

RewriteEngine on

RewriteRule /brand/(.*)/ products.php?BrandID=$1

I would highly recomment the Yoast SEO plugin, and would recommend the use of breadcrumbs within the template. In the header.php of your theme you I would suggest you add a rel="noindex, follow" on category pages. I wrote an article detailing how to set this up, and so far its working well for me: http://www.laceytechsolutions.co.uk/blog/wordpress-development/avoiding-duplicate-content-in-wordpress

On another note, I have used a plugin called GigPress, for an event website I put together and this works wonders, I would highly recomment this plugin. I hope some of this information is useful to you.