OK, so here’s a quick summary. Various assistants enhancements, including the very useful duplicate button, had been contributed by @Arc_Mantis.
Thread - New Krita Assistant Tool Features - Develop - Krita Artists
MR - Assistant Tool New features: Editor Widget Redesign and Duplication Function (!1966) · Merge requests · Graphics / Krita · GitLab
Again, you will find this available in Krita Next Nightly build, which you can get from the official page. Nightly builds are bleeding edge and may be less stable than the released version.