“They're one in the same person.”
Grammar Entry 652 / 1605 60-second read
One in the same vs. One and the same
Set phrase.
The comparisoni
“They're one and the same person.”
The ruleii
¶
ONE AND THE SAME.
The set phrase is "one and the same" — with AND, not IN.
Memory aidiii
Remember it like this
AND the same.