I saw there are plenty of similar packages on debian-netinst.iso, in
deb form. A
udeb is a stripped down
deb; why are there duplicates of full and "slim" versions?
They aren’t really duplicates, they have different purposes.
udeb packages are stripped-down packages containing only the minimal amount of files required for the installer. They are used to run the system during its installation. The idea behind these minimal packages is to reduce the run-time requirements of the installer.
deb packages in the installer images are full-blown packages and are installed during installation.
Some packages appear in both forms in the installer because they are both used by the installer and part of the installation. For example,
btrfs-progs appears as a
udeb package because it can be used during installation (to create Btrfs volumes); it also appears as a
deb package because it has to be installed on systems using Btrfs volumes.