Why "the one of the" is a common grammar slip

What kind of error is it?

Grammatical error related to redundancy

Why do people make this mistake?

People may make this error due to confusion or a slip of the tongue when trying to specify that something is part of a group or category, but it results in redundant wording.

What is correct?

Use either "one of the" or "the one" depending on the context. "One of the" is used when referring to an individual item among many, while "the one" is used for emphasis or clarification about a specific item.