I refuse to believe that that is a thing.

I can't even make sense of the last sentence and English is my best subject

*secretly proud of herself for understand the whole thing without using Google translate*

I hate English

You can usually replace the second "that" with "the", a proper noun, or remove it altogether. In fact, "that" is actually a pretty superfluous word, it just take more time and energy to find grammatical alternatives.

I had has no idea that that that that thing was such a bad thing