Deadmanwalking is correct. Getting the skill trained twice does not increase its proficiency to expert unless something specifically says otherwise (as is the case with many archetype dedication feats).
In the case where you would get the same skill twice from a background and a class at level 1, you instead get to choose a different skill.
For example, if you got Stealth from both your class and your background, you would instead get Stealth plus one other skill of your choice.
This only applies at first level though. If you end up with duplicate skills later, refer to the ability giving you the skill to see what to do. If it doesn't say you get another skill in the case of duplicates, you don't, and your duplicate skill is wasted.