My point is that the instructions page is still rather sloppy about the Linux installation, telling too little and too much at the same time.
Ah, now that I made the same search, I realize that this is often reported as a problem with apt, when actually it is a something that package creators should take heed of and be more careful in how to create their packages. The methods to get rid of the message lead users to open big holes in their security. So I really should report a bug on this to duplicati team.