We have an eCommerce site we have built for a customer and the products are allowed to appear in more than one product category within the web site. Now I know this is a bad idea from a duplicate content point of view, But we are going to allow the customer to select which out of the multiple categories the product appears in will be the default category. This will mean we will have a way of defining what the default url is for a product. So am I correct in thinking all the other urls where the product appears we should add a rel canonical to these pages pointing to the default url to stop duplicate content? Is this the best way?

Many of our e-com sites we build for customers have drop down menus to help the user easily find products without having to click - Example: http://www.customandcommercial.com/ But this then causes the report to trigger too many on page links We do have a site map and a google site map So should I put code in place not to follow the drop down menu link items or leave in place?

I have a customer site where you can search for products they sell url format is: domainname/search/keywords/ keywords being what the user has searched for. This means the number of pages can be limitless as the client has over 7500 products. or should I simply rel canonical the search page or simply no follow it?