Answer: "one and the same" or "one in the same"?

[green_grrl]

English is full of phrases that get sprinkled through conversation. We pick them up, use them, and don't think too hard about them until we have to write them down. Then we realize we're not quite sure what the exact phrase is. lauramcewan asked us about one of these: "Is it 'one and the same' or 'one in the same'?" When spoken, the phrase tends to sound like "one 'n' the same," so it is understandably confusing.

Unlike some other usage questions, there is only one right answer here. ( I'll illustrate with the Avengers. )