One in the same,' or 'one and the same'??
'One AND the same' is the one you want to use if you want to emphasize sameness.
'One AND the same' is the one you want to use if you want to emphasize sameness.
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There is no difference between one and the other. They are one.
I first heard someone use the second one about 30 years ago and had never heard it before but they insisted that was the "correct" term.