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: