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.