Thinking about the kit we are going to need collectively rather than individually wondering if it was an idea to get a list of what we're taking and then decide if we really need to buy/take another.
Good question though, although despite the stipulation that no-one ever goes off on their own, it's still possible that two could disappear and then find they need the thing that's in someone else's pannier, so I'd be wary of relying on the fact that "Fred's got tyre levers" when Fred's nowhere to be seen and your back tyre's in shreds.
I'm not saying it's not a good idea, but can't immediately think of something that we'd only need one of between the lot of us.