This question already has an answer here:
Scenario : I make a character (rogue) and put proficiency in acrobatics, at level 1 I get expertise and double my acrobatics proficiency buff. (text from roll20):
Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.
At level 4, I then decide to take the Acrobat feat:
You gain proficiency in the Acrobatics skill. If you are already proficient in the skill, you add double your proficiency bonus to checks you make with it.
They are worded differently, the expertise class skill says to double my existing proficiency bonus before adding it while the acrobat feat says I get to double what I add to my skill check proficiency bonus.
Does this mean I can get +8 to my proficiency modifier or no? I looked at past related topics and all of them seemed to be asking about doing this through 2 class abilities by dual classing, save for one. That one was labeled a duplicate and unanswered and then linked to an old question about using 2 different class abilities to meet a similar result. This is through 1 class ability and then a feat. Does that not make a difference?
The equation is the same regardless of how it'd be ordered correct?
1d20 + (3 ability score + ((2 prof + 2 feat) × 2 expertise) or
1d20 + (3 ability score + ((2 prof × 2 expertise) + 4 feat)
The common answer is "you still only add it once and multiply it once" If this is correct then how is it not doable? Expertise doubles you bonus while the feat adds it twice. Is that not different to doubling it twice?
The end result is Expertise makes your proficiency bonus double where applied for that skill by default, the feat specifically states to add that proficiency bonus to the skill twice. Is that not how it is worded?
I do not think Jeremy Crawford's answer that is being quoted is correct in this scenario; you are not multiplying the proficiency twice, you are multiplying it once(which doubles the prof bonus) and then add that result twice as the feat says you do.
If you can not do this, then that would mean the feat is not able to apply regardless of having expertise because the feat specifically states for you to add your proficiency modifier twice to your roll.