[Grammar] I'm not sure "I know" what you're getting at.

[mokbon]

Hello

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Mrs. Geller: Tell her what?

Monica: How she drove you crazy, picking on every little detail, like your hair... for example.

Mrs. Geller: I'm not sure I know what you're getting at.
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Friends s1, ep8

Are these two the same?
1. I'm not sure what you're getting at.
2. I'm not sure I know what you're getting at.

Then, why does this add "I know"?
It makes me misunderstand the meaning.

 

[Coolfootluke]

I am not a teacher.

I wouldn't say they are the same. They can mean the same thing, but the first is rather telegraphic. We can leave words out, but beyond a certain point it becomes poor style. "I'm not sure what you're getting at" is a too-short form of "I'm not sure I know what you're getting at."

 

[mykwyner]

Hello

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Mrs. Geller: Tell her what?

Monica: How she drove you crazy, picking on every little detail, like your hair... for example.

Mrs. Geller: I'm not sure I know what you're getting at.
--------------------------------------------------------------------
Friends s1, ep8

Are these two the same?
1. I'm not sure what you're getting at.
2. I'm not sure I know what you're getting at.

Then, why does this add "I know"?
It makes me misunderstand the meaning.

A number of different verbs could be used to get the essentially the same meaning in this sentence.

I'm not sure I understand what you're getting at.
I'm not sure I follow what you're getting at.
I'm not sure I see what you're getting at.

The difference between your sentence 1 and 2 is that the "I know" puts more emphasis on the listener's lack of knowledge of the situation being referred to.

Overall, if the speaker had omitted the "I know" from the original utterance, the meaning would have been the same.