Jimmy Wong <wyuenho@gmail.com> writes:

> 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?

The issue is that we want to go through package--save-selected-packages,
that is given a new value to assign to `package-selected-packages'.
An otherwise, I my understanding is that add-to-list is not conventional
in executed code.

The alternative is to check for duplicates before invoking the function: