I recently wrote about Gregory Batesons definition of information: ‘...what we mean by information—the elementary unit of information—is a difference which makes a difference...’. I refered to this as not even a tautology (the saying of the same thing twice over using different words). I was right, it isn't a tautology.
The phrase 'a difference which makes a difference' is a diacope. This is a literary device formed by the repetition of a word or phrase before and after an intervening word or phrase.
The canonical example of a diacope is in Hamlet: '...to be, or not to be!'
The OED describes it as ‘A figure by which two words that naturally stand together, especially two parts of a compound word, are separated by the intervention of another word’ (Webster 1864). Another famous example is the first sentence of Tolstoy's Anna Karenina: 'Happy families are all alike; every unhappy family is unhappy in its own way'.
Still, my favourite is Bateson's.