Here is a learning support on using a comma between repeated words like is is, in in, and that that. This is the ninth of fifteen related learning supports on the use of the comma. (You can find an excursus on this choice of publishing practice here.)
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.