What do you do when you are given something as a career skill twice? I’m realms of terrinoth the Deep Elves start with Knowledge Forbidden as a career skill (they also get two free ranks in it and can’t raise it past that at character gen). If I then choose to play as a Mage, I also get that skill from there. This can also occur when taking talents.

My question is, do I just loose a benefit as something can only be a career skill once? I cant find anything telling me what to do in this instance.

In those cases, it only matters during character creation, where a character could, theoretically, get 2 free ranks in a skill. Otherwise, you cannot get a career skill twice, so nothing happens.

Since not all races provide the benefit of an additional career skill and/or additional ranks, you cannot generalize the issue.

In the case of the Deep Elves, the skill is a special feature and taking the Mage career afterwards has no mechanical impact nor benefit as @Swordbreaker already mentioned.