Christo said:
recently I have noticed that some people use an adjective twice instead of using an adverb like 'very'. For example,
"Bill is off work today because he is ill but he isn't ill ill."
Is there name for this? Is it a recent thing? Is it regional?
I agree with the other posts.
First of all, there is a technical term for it--I just can't remember it at the moment. Sorry

(Hopefully, Mike with know.

) Second of all, it's not a recent thing, nor is it regional.

Restating a word is often used to emphasize (i.e. very) and even de-emphasize (i.e. not very) the meaning of the word.
It expresses,
(not) in the full sense of the meaning.
He is not ill, ill.
(He is ill but not in the full sense of the meaning of the word ill)
He is ill, ill.
(He is ill in the full sense of the meaning of the word ill)
All the best,