Getting something like Proficiency or Advantage or Resistance multiple times does nothing in D&D5.
For example if you make Karlach a Wildheart Bear Barbarian, during the rage she still only has resistance to fire damage, but not immunity, or that she would only receive 25% damage from fire, or anything like that.
But you can stack Proficiency by picking Expertise, which indeed gives you double Proficiency bonus on a skill.
With no guarantee of completeness:
- Rogue level 1 and 6 as well as Bard level 3 and 10 can pick Expertise for two skills on each occasion, i.e. both end up with 4 experises
- The Actor feat gives you Expertise in both Deception and Performance
- Rock Gnomes get Experitse in History
- Knowledge Domain Clerics can pick expertise of 2 out of 4 skills: Arcana, History, Nature, Religion
So you can make a Gnome Rogue(6)/Bard(3)/Knowledge Domain Cleric(1) and have 9 Expertises, which is half of the skills you can have. If its a lore Bard, you also get 8 Proficiencies (4 from Rogue, 1 from multiclass Bard, 3 from College of Lore), too. Though since Rogue and Bard can only pick Expertise in skills they have Proficiency in (which applies neither to the Actor feat nor to Rock Gnomes nor to Knowledge Domain Clerics, who all gain Proficiency together with Expertise), you really would only get 4 more skills you still have proficiency in.
