Hi! I'm having exactly the same problem. I work with this project in which I have several AE compositions for animated elements I use repeatedly. For that reason, I often alt+drag them across the timeline. And they all behave as expected: duplicate on timeline, and not in the project panel. Except for this one (the intro/outro file).
Every time I alt+drag the intro, it duplicates the footage. Is that some kind of error?
Note: Ctrl+C/Ctrl+V does not afect it the same way. Neither does duplicating the whole sequence. This behaviour is produced exclusively while Alt+dragging, and also exclusively for this comp. All the others behave normally...
Note 2: I've used this group of AE Elements in several different Premiere Projects. And no matter how many new projects I create, this isolated footage alway behaves the same.