According to the Navy SEALs, "two is one and one is none" thus the need for redundancy. In my book (granted my book pertains to the US and many other first-world countries not the countryside of Afghanistan), if I am carrying two of the same thing I have one thing too many. I am a fan of ultralight backpacking, ultralight travel, and one-bag carry-ons, mostly because I have to schlep everything I am carrying myself and these old bones aren't getting any younger and I am way past the desire to carry a 60 pound pack.
As for packs, I have everything from a three-liter running vest to a 55l backpack and several sizes in between. As for stuff to put into these packs, I don't like redundancy of items (I do like items that have multiple uses, however). I don't want to have to change the batteries every six months in five different flashlights. I don't want to have to rotate food out of several different bags every few months. This means that instead of having several different packed bags, I have several bags and several single-purpose interchangeable component bags.
Here's how my gear closet looks:
- Pick a bag: running vest, messenger bag, daypack, 30 l daypack for travel, 55 l backpack for a long-distance backpacking trip or bug out situation.
- Put all of my EDC stuff in the bag (this stuff is carried whether I am going out for a run or a day hike or hopping on a plane to go on vacation).
- Pick appropriate shoes, clothes and outerwear to wear (like things are stacked together--running socks and clothes, backpacking clothes, travel clothes, formal clothes, etc). Put additional clothes or shoes in large ziploc bags and add them to the bag if this will be a multi-day trip.
- Pick appropriate components: tech bag, outdoor survival kit, toiletries kit, international travel kit, office kit, first aid kit, backpacking gear (tent/stove/sleeping bag/etc)
- Hit up the safe for: cash, firearm/holster/ammo
- Run by the pantry for: water, food (grab-and-go food for running and backpacking has its own section in the pantry)
With this system I can literally be packed and out of the house in less than 10 minutes whether I am heading out to a meeting or leaving for a 10-day backpacking trip. The items in my gear closet have been reviewed and refined over the year, and every so often I take my supplies list and make sure I have everything on the list and/or change the list if needed (this is also the time to change batteries in the flashlight, sharpen the knife blade, change batteries in the radio, etc).
One of the most important aspects of this system is to replace anything that has been used as soon as I get home so everything is ready to go for the next trip and I don't have to wonder if I am missing a toiletry kit item or used all of the moleskin in the first aid kit on my last run.