I don’t think you should dedup the variable that could have been modified by something else such as package-install. This may make debugging harder should package.el itself introduce a bug that duplicates pacakages in the variable. How about just using good old add-to-list?
On 8 May 2023 at 1:03 PM +0100, Eli Zaretskii <eliz@gnu.org>, wrote:
> > Cc: 63338@debbugs.gnu.org
> > From: Philip Kaludercic <philipk@posteo.net>
> > Date: Mon, 08 May 2023 10:36:55 +0000
> >
> > Jimmy Yuen Ho Wong <wyuenho@gmail.com> writes:
> >
> > > Reproduction:
> > >
> > > 0. (setq custom-file (const user-emacs-directory "custom.el"))
> > > 1. M-x package-vc-install company
> > > 2. M-x package-vc-install company RET y
> > > 3. C-x C-f ~/.emacs/custom.el
> > > 4. Observe that `company` has been listed twice under
> > > `package-selected-packages`.
> > >
> > > Expectation:
> > >
> > > Installing the same package twice should not result in its duplication
> > > in `package-selected-packages`.
> >
> > An easy fix would be just to ensure that package-selected-packages is
> > always deduplicated before assigning the value:
>
> This is OK for the emacs-29 branch, thanks.