It's a very poorly worded warning. I wish that the Canvas developers would clarify it. Really, very poorly worded.
Here's what I think it means: Let's say you import a quiz from Course 2021 into Course 2022 and edit the quiz in Course 2022. Then, if you try to import the same quiz from Course 2021 into Course 2022, the edited version will be lost.
My recommendation to Canvas is to add the word "target." That would give us "Importing the same course content more than once will overwrite any existing content in the TARGET course."
That being said, you're right that the word "any" is still problematic. That implies that all of the existing content in the target course could be overwritten. Again, this is a big fail on the part of the Canvas developers.