I would like to know how many users try put this "$$sale.com" to browser.
And answer to your question. It is my subjective point of view.
I think that its better to use just 1 character - no doubles. But it is not cache as in your example. That is another story.
Reason why just 1 character is because minimalise typos. And make shorter, more memorable address.
Its great question thanks for opening such thread.