Technically speaking appimages are compressed files so maybe someone clever can come up with an idea on how to dedupe them. With mass adoption this issue i think may be solved by someone concerned about disk space. Personally, and probably many casual users, are not as worried about space as they are about convenience. But I think your point is valid, and I think this would make an interesting side project for someone out there. For me, I'd like to see more adoption. The nice to haves will follow.