One and the same is the logical formulation of the expression meaning the same person or thing. This expression is not hard to parse; it uses redundancy (one and the same being synonyms) for emphasis. The eggcorn one in the same sort of makes sense—if we imagine something being inside the same thing as itself—but it’s not the standard phrase and is widely viewed as a misspelling.
True nuts don’t split—the seed and the fruit are one and the same. [Telegraph]
To them, efficiency and bureaucracy are one and the same. [Daily Telegraph]
Tea Party Republicans are itching for a fight on these issues — for them, the culture war and the budget showdown are one and the same. [Salon]