My character has proficiency with an instrument, she also has expertise with in performance. I know the rules say you add and multiple proficiency once.
The RAW for instrument proficiency states
Musical Instrument: Several of the most common types of musical
instruments are shown on the table as examples. If you have
proficiency with a given musical instrument, you can add your
proficiency bonus to any Ability Checks you make to play music with
the instrument. A bard can use a musical instrument as a spellcasting
focus. Each type of musical instrument requires a separate
proficiency.
Expertise states
Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.
At first, I questioned if this meant I don't roll a performance check for how well I played my instrument. Similar to how picking a lock has you roll dexterity + proficiency / expertise (but only if you are proficient in thieves tools).
Yesterday me and my GM found two different contradicting Sage Advice answers.
This first link here, states that the GM decides whether they use performance or the tool proficiency, or the DM can allow the player to choose which proficiency to use.
But this second one here, states that the player uses both on the same roll.
They are both written by the same person. The RAW and the Sage Advice tweets from Mike Mearls seem to contradict themselves.
My DM and I tried to figure it out, but neither of us could come up with something that did not seem like making it too effective or too ineffective.
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Using both would mean I could get expertise twice
We agreed while that'd amount to next to nothing in a combat scenario, on the role play side that would mean my minimum roll at level 3 with a 16 charisma is a 15. It could be argued it balances out since I am only that skilled in that instrument, and it eats up an extra expertise choice.
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Even without that second expertise it is still a minimum roll of 13.
It kinda makes sense that someone who has put that much effort into their instrument of choice would never preform less than a 10.
When playing an instrument, do you use your instrument proficiency bonus, your performance skill bonus, or both?
I asked this question a few months back, but it was closed because I could not correctly get across what I was asking about. Now I think I can.
Best Answer
Reference the tool proficiency optional rules from Xanathar's Guide to Everything
You can't apply your proficiency bonus more than once to a single roll, as you've established; it seems like you should probably make this roll using your expertise in performance, since performance is a relevant skill and with expertise that grants you the best modifier. However, there are options to represent your particular combination of proficiencies beyond simply adding more bonuses to the roll.
Xanathar's Guide to Everything, in the Dungeon Master's Tools chapter, includes a section titled Tools and Skills Together, which addresses exactly this situation where a character might have overlapping tool and skill proficiencies. It makes a couple of suggestions about how a DM could adjudicate the situation:
The chapter then goes on to describe the possible benefits of various tool proficiencies, and gaining advantage on a performance check incorporating that instrument is given as a benefit of proficiency with a musical instrument:
An extra benefit to a successful check might perhaps be allowing the bard an automatic success (or at least granting advantage) on a subsequent social skill check to influence the audience after the performance.
The intent of these optional rules from Xanathar's is to make tool proficiencies more valuable, in order to encourage players to take and make use of them as character options, and reward characters who have invested in such specialisation.