Update:The contents of this post are a work of fiction, for amusement purposes only.

Any correspondence between concepts and constructs it contains and any real literary entities of similar name or form are purely coincidental, as the author hasn't got a clue!

That that is one of the words that can be grammatically correctly abutted within a sentence is no surprise, as just demonstrated.

And and can also be so abutted.

In these usages of that that, and and and, the first repetition is referring directly to the second.

But that that that and and and, are not the end to these linguistic anomalies.

He said the matter was closed, and that that should be an end to it.

In this usage, the first that is not referring to the second that.

Once the sentence itself starts both using this phenomena and referring to it, the 'that that', that that 'that that' is referring to can itself become self-referential, with the consequence that that, that that, that that, that that refers to, tends to become obscured.

However, is it a matter of speculation whether is, is another word that can be so abutted?

Apparently not :)