A t-shirt is a unisex item of clothing. Same with things like socks and sneakers. When assigning categories, where do you put them? Under the “Male Clothing” category or the “Female Clothing” category? Or do you create a unisex category of their own?
In a case like this, double categorization is the best solution. The same t-shirt should be accessed by navigating through either the male or female category.
T-shirts and socks aren’t the only items that can defy strict categorization. Take a microwave, for example. Does it belong in “Home Appliances” or in “Electronics?” Take butter too. Where would you categorize butter? Is it under “Spreads” with peanut butter, margarine, and apricot jam or under “Dairy” with chocolate milk, yogurt, and cheese? What about broccoli? Where does it belong? Is it with the other vegetables or with the other implements of torture in the BDSM section?
No matter your personal opinion, you will find dozens of other people who hold the opposing one. The simplest thing to do for a smoother user experience is to put such products in the two or more categories to which they claim membership— except broccoli. Everybody insists on classifying it as a vegetable for some reason.
Have a single URL
Double categorization comes with one major caveat: don’t duplicate the product listing. This might complicate stock keeping by making a single product look like two separate ones to your inventory management system. The cleaner way is to host the product on a single URL and then make it possible for users to navigate to it from two or more categories.
Avoid double categorizing products just for the sake of it
Anybody with more than a cursory knowledge of SEO knows that internal links are great for your ranking. This might tempt you to double, triple, and quadruple categorize everything you can. Don’t.
Don’t put a product in a category to which it doesn’t belong in the hopes of ranking higher on Google. A knife could be listed under “Utensils,” “Weapons,” and “Tools” but it doesn’t belong in “Toys.”
Take double categorization too far and your categories will stop meaning anything, making your navigation horrendous and disappointing all the shoppers in your store. Terrible navigation kills any SEO benefits you gain from your internal linking. But as long as you can do it without breaking your internal navigation, go nuts.