I have backups of a few really special eyeshadows, pigments, and lipsticks, but only one backup of each product.
I don't wear makeup every day, and MAC products tend to go so far, that I feel secure just buying one backup.
I don't blame you a
bit for buying multiples of something you love, unless you used your insulin money, or mugged a little old lady.
As long as you've got the funds and the storage space, I don't see why you shouldn't buy multiples.
If you get tired of them, or MAC comes out with something you like even better, you can always sell them, or swap or give them away.