A recent Lifehacker article suggests you should buy two (or more) of everything in order to avoid having to pack them when you go out of the house.
The idea of having two of everything just because seems to be rather fiscally irresponsible in the first place. But makeup? Has the author ever stepped foot in a Sephora? Getting two of everything that I might want to touch up outside the house would cost a bloody fortune! I don’t even wear that much makeup and I’d still be looking at hundreds of dollars.