Right next to my house (Ofallon, MO) there are two grocery stores within 200ft of each other. This is just ancedotal but any time I've thought about this topic I've always thought about it in relation to those grocery stores.

I do some of my shopping in the corner store, and some in the nearby business cluster that has, among others, 4 supermarkets. I'd guess real estate, loyalty programs and product offerings outweigh proximity, in the case of foodstuffs.