This can happen if you have an exploded product feed without a product key. Simply, we are running through the feed row by row and showing the products that match the business rules. If the feed contains the same product 6 times (say in 6 different sizes) we will could show the product 6 times. In this case you need to apply a distinct key to your content source.
A distinct key is applied when you wish to only show one of an item when it may be listed in the feed multiple times. For example, if you are a fashion retailer you may have a dress called 'Floral Summer Dress' in the title column - this dress may be listed in the feed in 6 different rows as you have it in 6 different sizes. However you don't want to show it 6 times in the email. Therefore you would apply a distinct key on 'title'. We would then only show one of each product with the same title in the email.
If the 'Floral Summer Dress' was available in multiple colours you could apply two keys - one on title and one on colour. This means that we will show one of 'Floral Summer Dress' in each of the colours it is available in.
To apply a distinct key to a feed, navigate to the content sources area of the platform and edit the content source you would like to apply the key to. Click through the wizard until you reach the key section, click on the triangle icon to expand the feed, and select the field you wish to use as your distinct key. Save your changes.
The update may not happen immediately, wait a couple on minutes and then refresh your email/tag. The duplication will no longer be visible.