Have you ever had a case where you felt compelled to include strange things like a double that in a sentence? If so, then what did you do to resolve this?
For me, I never knew whether it was acceptable grammar. However, what I did learn was that it was a logic distractor, could lead to confusion, and therefore should be reworded to avoid this.
There are three very different uses of that:
Subordinating that: “I know that this is the answer.”
Demonstrative pronoun that: “That is not the answer.”
Adjectival that: “That answer is not it.”
Double that occurs because the first that is the subordinating that, and the second that is a demonstrative pronoun or adjectival that. That is, if you subordinate a clause that begins with pronominal or adjectival that with that, you get that that, as in “you know that that that that from the previous sentence was different from the one in this sentence.”