I've just updated to 1.5.8. Everything went as usual, and no errors, but now, when I view a category, some of the products in the category view are listed two or three times. This isn't a duplication of products. There is actually only one product, and both instances of the product go to the exact same productID. This is strictly an issue with how the list is being rendered. for example: let's say in Category 1 there are 6 products with prod IDs 1-6. When I view category 1 from the menu, the result looks like this: product 1, product 2, product 3, product 3, product 4, product 5, product 6. (in this example, product 3 is duplicated in the list. If I click on either of them, they go to the exact same product ID (which makes sense, because in the DB, there is only one "product 3"). But why is product 3 showing up twice on the list? I don't know how to fix it.

This is happening in more than one category.
This is happening with random, multiple products (with no similarity or pattern between the products)
I've checked, and all products have valid master cat IDs, product types, and other information
This isn't happening in the 1.5.7 live store
This happens with every template, including the classic and classic_responsive

Any ideas on how to fix it or where to look would be greatly appreciated.

If it's relevant, I have a lot of categories/subcats and over 1000 product IDs. There are no products in top categories or anywhere they shouldn't be, as far as I can tell. Thanks.