The established English phrase is “one and the same”. It means that two names, descriptions, or roles refer to exactly the same person or thing.
For example, “The owner and the manager are one and the same” means the owner and the manager are not two different people. They are the same person.
You may also see it in sentences like these:
- “Her biggest supporter and her toughest coach were one and the same.”
- “The town founder and first mayor were one and the same.”
The version “one in the same” is common in speech and informal writing, but it is generally treated as a mistake or a nonstandard variant. In careful writing, “one and the same” is still the preferred form.
If you want the standard expression, use “one and the same” when two labels point to the very same person or thing.
