Categorize everything and make a hiearchy (tree) to represent each product.
You already have your domain. Here I'm gonna call it example.com. The next text that should follow is any major category where many of your products fall under.
So if you ran a store, and it only sold a wide variety of items but a third are premium items and a third are standard items and a third are discounted items, then the first text could be one of those three things I mentioned and you can begin your tree like this:
Do keep in mind that each URL needs to have unique content. For example, http://example.com can talk about a few best sellers and why they sell well.
could present information about what it means when products are premium and then example products can be displayed.
Then you can keep going deeper. So if you sell premium forks, premium plates, discounted cars, discounted apples, standard paper, and a standard computer, then you can have URL's like this:
Then for each additional URL I added, you talk about each item and why its in that category. For example, You can talk about forks and why they are premium.
As a bonus, you can define a naming convention so that navigation keywords don't follow it. Take a look here:
As you can see, I added a colon in the name and defined a rule where an item category name or item name cannot contain a colon because if it does, then navigation could be messed up. As for the above URL's you could redirect
unless you're writing unique content for each page (meaning where the first URL is a nice intro and the pages list the content)
Nevertheless, make sure the content between each page is unique enough and use unique titles and unique meta descriptions and try to have 300 words per page.
Once you get into a habit of categorizing then you should never run into a duplicate issue again.