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. Sometimes it might be because the word being repeated has 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 second meaning.

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).