On p125, paragraph "Proficiencies" (that's in the Backgrounds section of chapter 4).
It says : "If a character would gain the same proficiency from two different sources, he or she can choose a different proficiency of the same kind (skill or tool) instead."
So if you're a Wood Elf and your background is Criminal, instead of gaining proficiency with the skill Stealth a second time, you can pick any other skill.
