A typical conversation among members of my age demographic could go like this:
Person 1: Did you know that x > y?!?
Person 2: Are you sure?
Person 1: Yeah, I'm sure.
Person 2: Are you sure sure?
Is this grammatically correct?
Learn English – “Are you sure sure” — is this repetition grammatically correct
adjectivesadverbsgrammaticalityrepetition
Related Solutions
Both of these are acceptable, though the focus in each is different.
By placing different parts of sentence at the front, you make them more important.
In the first sentence, the focus is on communication and the inability to do it. You could extend this as follows:
We could not communicate through the phone, though we could use it to (do something else).
In the second, the focus is on the phone and its uselessness in the situation. It could be extended thus:
Through the phone, we could not communicate but through the (something else), we could.
This works when written. However, in spoken language, stress and intonation could alter the focus regardless of word order.
As a matter of preference, I would tend toward the first example, simply because it is the natural order of the sentence without clauses being juggled and extra punctuation being added. I would only choose the second if I specifically wanted to alter the focus, as explained above.
Hope that helps.
Yes, the sentence is grammatically correct; here is a syntax diagram (parse tree) for the sentence: 
Best Answer
I don't think "grammatically correct" is really a meaningful issue in respect of this relatively common spoken usage - but if I have to have an opinion, I'd say it's valid but informal.
In general, to be a [noun] noun], or be [adjective] [adjective] is simply an informal way of adding emphasis.
Often it's because the word being repeated has acquired multiple shades of meaning - arguably in OP's example the word sure can mean anything from "Okay, I guess so" to "I would stake my life on it". The repetition is intended to focus attention on the primary meaning (certainty).