You are one or you are a one, which one is correct?

Should I use 'a' before the word 'one'? Is it grammatically correct?

For example:

There are many intelligent people, tell me if you are one.

There are many intelligent people, tell me if you are a one.

Which one is correct?

Also same question arises when I try to say something like "I am (a) one"

Solution 1:

"You are one [of them]" - of a type of person previously mentioned. This is standard English.

"You are a one!" (without any previous mention of a type of person) is a rather old-fashioned British expression of amused reproach. See https://www.ldoceonline.com/dictionary/you-are-he-is-a-one