Cant you just tailor it to each product because you wont have 20 of the SAME product will you? when it would just be 1 listing.. Create a unique piece of each product you have

Is there other text on the pages that varies from page to page? If so, what percentage of the text remains the same from page to page? I think you only need to worry about duplicate content if the content is largely the same on each page, not just if some content repeats.

"Penalize" is probably not the right word. Google doesn't penalize your site in the case of duplicate content but will determine what it considers to be the main copy and will disregard the others if it considers them to be duplicates of the first. It is simply a matter of not wanting to return multiple copies of the same content in a user's search results.

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i think it's ok. it's part of the products description. i would not link in it. maybe to the brand's site but that's it.

Google may to see such things as duplicates, there are high possibilities
It is better to re-write such content and post on your website. I usually rewrite/ get some ideas and write on my own words when I do such promotions.

Google will not penalise your site. Duplicat content penalty is more with regards to duplicate content within the same site which is why cannocial tags are used. Duplicate content across different sites is content syndication. As someone mentioned google disregards multiple copies from the serps but doesn't penalise

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I think that's all right. Because it's part of the products description.

Yes it will be duplicate content, but you are describing a specific product in that content. So you can use it 20 times if it is just a paragraph. And if you want to stay safe forever, you need to change some lines in that paragraph in two or three places to avoid exact duplicacy.