I understand what you're asking for, but it's ultimately a non-issue.

For content which can be done in the Duty Finder it simply isn't difficult enough for this to matter. Having multiple people playing the same class is not optimal, but it affects Duty Finder content so minimally that it just isn't a problem worth considering.

In harder content any party worth a damn isn't doubling up on any job anyway.

If you're simply putting this forward as a personal annoyance your feelings are valid, but you have to understand that's what it is. A personal problem. It's not worth increasing queue times while the Duty Finder tries wait for a unique job. It's not worth the dev time to look into a way to separate players who have it enabled with players who don't and deal with the alternate problems it would cause.

As someone already said if it really bothers you that much then the option to fix it already exists. Create your own parties in the PF.