If you multiclass correctly so you get expertise from both bard and rogue(e.g. Rogue 1 and Bard 3), and if in both classes you put the expertise into Deception, does it stack on an ability check?
Example:
The bard/rogue is proficient in Deception. With a 16 (+3) charisma at level 4 would the character get a +15 bonus to Deception rolls?
From the basic rules:
Occasionally, your proficiency bonus might be multiplied or divided (doubled or halved, for example) before you apply it. For example, the rogue’s Expertise feature doubles the proficiency bonus for certain ability checks. If a circumstance suggests that your proficiency bonus applies more than once to the same roll, you still add it only once and multiply or divide it only once.
Emphasis mine. Even if you somehow took proficiency in the same skill twice (I believe the wording in the official PHB always says choose more or additional skills), it wouldn't apply because you can't benefit from a proficiency bonus more than once.
So, expertise doubles you proficiency bonus, not the stat modifiers themselves. lets say you have a level 3 bard and you choose performance for one of your doubles, then when you perform, you get a +3 from your charisma (Ex.) and a +4 from your proficiency, which would normally be +2.
