The cool thing is is that ...
Link: @5:11 of this video.
I hear many native English speakers repeat the word "is" when they say sentences like that. I don't think it's related to the grammar. I feel like it's a more of a filler word and it's not really needed. At the same time, it's not a stuttering because many native English speakers do it. Why do they repeat it? Is it related to a specific accent?