Have you ever met human beings that constantly reuse a certain idiom/figure of speech/linguistic pattern?

The valley girl using "like" every other word, for example?

Or I had a colleague who would use the expression "we can say" (in French, because we were speaking in French) basically every couple sentences for a bit.

Humans also repeat speech/linguistic patterns, therefore "repetition of the same pattern" is not sufficient to mark text as produced by AI :)

Yes but there are a lot more "idiom personalities" in humans (you just mentioned several) than there is in AI. Basically every English-language interaction with AI anywhere in the world produces more or less the same argot and style. Its like (heh) we're all talking to the same valley girl stereotype.