You can get rid of the duplicate products and categories in Google by adding a ‘canonical URL’ link in your Miva Merchant head tags.

You may have noticed that google has duplicates of some of your products and category pages but the pages have different URL’s.

A scenario where you would have duplicate content with different URL’s is when you have implemented short links into your store. You would have your short link similar to this: http://morningglory.mivamerchant55.com/catalog/lemons.html and then you would have the original long Miva Merchant link that looks like this: http://morningglory.mivamerchant55.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=mg&Category_Code=fruit&Product_Code=lemons
If you visit either link you will see that they bring you to the same page and this gets confusing for search engines and it also splits your page rank between the two.

Google has devised an element or tag if you wish to call it that which can be inserted into the head section of your category and product page templates that will tell the search engine which URL should be the used when there are multiple options and this is called the canonical URL. It is acceptable to make a canonical link point to itself.
The canonical URL would go in the ‘head’ tag not in the ‘header’ section of Miva Merchant.  Here is an example of what a canonical URL should look like.

<link rel=”canonical” href=”http://www.morditech.com” />

You can find a step by step on how to add canonical URL’s Miva Merchant here.

Matt Cutts, Google engineer, published a 20 minute video explaining methods to avoid duplicate content including the canonical link element and 301 redirects. He explains in vivid detail how Google treats duplicate content using the methods he describes.