I haven’t gone through to count, but I think it’s fairly common.
Yes. In the example in post 18, I demonstrated going from Emphasis to Advantage, back to Emphasis, and then to Disadvantage.
As I say in that post, what’s happening under the hood here is functionally the same as counting the number of sources of Advantage and Disadvantage, and applying Emphasis if they are equal, or otherwise applying the modifier there is a greater number of sources of. However, this rule achieves that functionality by the trinary logic of changing the status of the roll, rather than by counting.