Is It "One and the Same" or "One in the Same"?

One and the same is an idiom that means “very much the same."

When combined, these parts literally mean "the same and the same," figuratively showing how there are two separate people or things, but they are both the same.

• The letter in question and the one I bring today are one and the same.
• My character and I are really one and the same person.
• I think the people we’re talking about are actually one and the same.
• If their goals were one and the same it would be a more satisfying ending.

One in the same is a misstated version (known as a malapropism or an eggcorn) of one and the same.

Unless the phrase describes two identical things with one inside the other, one in the same doesn't make sense. Use one and the same instead.

• Incorrect: My sister’s opinion and mine are one in the same.
  (Our opinion is inside something else)
• Correct: My sister’s opinion and mine are one and the same.
  (We have the same opinion)

Even though it’s technically incorrect, one in the same has been around almost as long as one and the same has. In most informal cases, you could probably use one in the same (especially verbally), and people would understand you.

Some people further misuse the phrase by saying one of the same, meaning “another copy of something.” Unless you’re literally talking about making a copy of something, it’s an incorrect usage of the idiom one and the same.

• I like that cupcake you ordered. I’d like one of the same.
• My phone broke, so I bought one of the same.

A concept you can use to remember the difference between one and the same and one in the same is relating them to dolls. 

• When you have three of the same doll, they’re one and the same.
• Russian nesting dolls (Matryoshka dolls) are one in the same.
• When you want another one of the exact same doll, you buy one of the same.