Also if you find a clothing item you really like, it's often a good idea to buy more than one, because clothing (/shoes/etc) is more likely to go out of production than most types of things, due to Fashion.
Shoes might be the one item that you can't do this for, but maybe I'm wrong.
Pre-covid, I bought a pair of shoes. But I am averse to throwing items away before they are properly "dead" (or until they really require effort), so shoes that still have some soles on them (and fit comfortably rather than the tight fit of new shoes) would still be used.
Because there were lockdowns and the like, I wore my shoes a lot less. I went out a lot less. My grocery store is rather close to my house. The new shoes spent two years in their shoe box.
Lockdowns were lifted and I started spending more time outdoors again. Within a month, my old shoes were deemed "properly dead", and I switched to the new shoes. But within two weeks, the leather on them started flaking off.
Maybe I bought poor quality shoes. Or maybe I didn't store my new shoes well. But I've got the feeling that shoes don't keep as well as most other clothing does, and that you won't get the full lifetime out of an old-new pair.
I have seen this happen with one pair of shoes; not sure of the material but they disintegrated while being worn for the first time out of lockdown. But I've successfully stored (or seen stored) lots and lots of other pairs of shoes where the material has held up fine, including leather, cloth, suede, and rubber.
Two is one, one is none. Backup items are a related topic.
A good heuristic for the first things you should buy backups for is the things that if they broke / got lost / were stolen, would be most disruptive to your life while you procure a new one. Some ideas:
Duplicate IDs are hard to come by. However, you might be able to have multiple different forms of ID (such as an ID card and a passport).
And when you travel, duplicate debit cards! I just opened a second free checking account at my bank to do this. Keep one in your carry-on/hotel room, and one when you go out on excursions. So if you get pick pocketed or the ATM refuses to return your card, you have a spare.
This also lets you use cash for all your transactions - so the only way you can be skimmed is at the ATM, nowhere else.
If you find yourself at a stage of life where you have multiple domiciles, I strongly recommend buying duplicates of literally everything that isn't literally one-of-a-kind. Travel where everything but the clothes on your back and your laptop is already pre-positioned at the other end is a treasure.
Gum
Sometimes I need to ensure the quality of my breath, so I reach for a stick of gum. However, I don't always have my gum on me. Often, I forget it at home, or at work, or in a jacket.
So I bought 5 packs of gum that I distributed to my house, my work, my backpack, and my jacket.
Charger
I like to co-work outside my house, which entails bringing my computer there. Unfortunately, my computer's battery lasts at most 2 hours. Also, my home setup is somewhat intricate, so getting my charger into my backpack is a pain in the ass.
So I bought another charger, and put it in my backpack.
Conclusion
I had an ugh field around buying duplicate items, telling myself I should have the willpower or memory to make sure I have what I need when I need it. Giving myself permission to buy duplicate items has made life a little easier, and now I'm on the lookout for more places where I can apply this. Maybe I'll have multiple copies of a book in different places.