Comment 25 for bug 918489

It escaped my attention at the time, but Ubuntu 18.04 released with both a version of duplicity that shows the new incremental-backups-also-have-this-issue behavior (see my comment 22) and a release of deja-dup that wasn't yet fixed to avoid it.

Which means that deja-dup in Ubuntu 18.04 is still affected by this bug (for incremental backups).

These two commits landed in deja-dup 39.1 and should work around it, if someone wanted to patch deja-dup in 18.04 (I've opened a target for bionic for this bug):
https://gitlab.gnome.org/World/deja-dup/-/commit/4f325940dae7fc259b4be70fccec40c94617f4d4
https://gitlab.gnome.org/World/deja-dup/-/commit/135f4c83774b6dafe194236f99f1405f45032498

For users, you can also install the snap version of deja-dup to avoid this as well.