Learn English – Usage of comparative with a set of adjectives

The utterance is

This [wig] is made out of the Yaki synthetic hair, not the typical synthetic hair. And whenever, just FYI, whenever you guys are doing, um, characters with really thick hair such as Rapunzel, get Yaki synthetic hair because it looks so much more realistic [than other synthetic hair]: there's no shine whatsoever to it, um, and it's so much more thicker [than other synthetic hair]. Um, so I really, really, really, really recommend this.

The woman was 21 when the video was published, and she seems to be quite aware of what she is saying and not stumbling for words. That she uses a so-called double comparative ("much more thicker") with aplomb is evidence that the form is grammatical for her, whose usage is either an idiolect or based on a dialect. In spontaneous spoken speech it cannot be called "ungrammatical" for those for whom it is grammatical.

Briefly, yes I have heard (and read) other native speakers of English use double comparisons. A somewhat famous instance in written English is

Clara Basil is the most strangest person I know.

This sentence, which appeared in the US newspaper The Atlanta Constitution has been referred to in several studies of the usage of the "double comparative" in both British and American English. It is a feature of some people's language. In the dialect called current standard (written) English, it is an error. It seems dialectal in both the US and the UK.