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Do features that allow you to add twice your proficiency bonus, under some other name, stack with Expertise?

For instance, dwarves' racial Stonecunning trait, with bards' Expertise as applied to History checks, to identify the origin of stonework.

The text of Stonecunning is reproduced below:

Whenever you make an Intelligence (History) check related to the origin of stonework, you are considered proficient in the History skill and add double your proficiency bonus to the check, instead of your normal proficiency bonus.

And, for good measure, the relevant portion of Expertise:

Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.

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Your proficiency bonus can only be added or multiplied once by any ability or combination of abilities

From the Basic Rules / PHB p. 173:

Your proficiency bonus can’t be added to a single die roll or other number more than once. For example, if two different rules say you can add your proficiency bonus to a Wisdom saving throw, you nevertheless add the bonus only once when you make the save.

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.

Thus, any features that would result in you multiplying or adding your proficiency bonus more than once would not stack - you only get to add or multiply or add your proficiency bonus the one time.

Example: Stonecunning and Expertise don't stack

In the case of your example, here is what they would both do to a roll:

proficient check with Expertise = die roll + ability mod + proficiency*2

Intelligence(History) check with Stonecunning = die roll + Int mod + proficiency*2

Note how both features do exactly the same thing: allow you to add double your proficiency bonus to a qualifying check.

Here is what it would look like if you let both apply to the same roll:

Intelligence(History) check with Stonecunning and Expertise = die roll + Int mod + proficiency*2*2

or

Intelligence(History) check with Stonecunning and Expertise = die roll + Int mod + proficiency*2 + proficiency*2

However you do the math, allowing both to apply to the same roll would result in either multiplying your proficiency bonus more than once or adding it more than once. Either way, it violates the rule and is thus not allowed.

Example: Mastery Ioun Stone and Expertise would stack

A rare few effects add a static bonus to your proficiency bonus. These additive effects are highly uncommon though. Most features multiply your proficiency bonus and thus fall into the previous example and do not qualify. An example of an additive effect is as follows:

The Mastery Ioun Stone says:

Your proficiency bonus increases by 1 while this pale green prism orbits your head.

Since this only adds one to your proficiency bonus, there is nothing preventing it from stacking with Expertise.

proficient check with Expertise and Mastery = die roll + ability mod + (proficiency+1)*2

You still only add your proficiency modifier once to the roll and you only multiply it once and thus they stack validly.

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    \$\begingroup\$ @KorvinStarmast: Turns out you were right! It's been asked generally once or twice, and specifically (about specific feature combinations) a few other times. \$\endgroup\$ – V2Blast Nov 18 '18 at 23:13
  • \$\begingroup\$ You "add it only once" and "multiply it only once"...so does that mean this combination does stack? Because Stonecunning affects what you add and Expertise affects whether it's doubled. \$\endgroup\$ – Stackstuck Nov 19 '18 at 17:13
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    \$\begingroup\$ The feature says: "add double your proficiency bonus to the check". Remember that proficiency bonus is always applied to a roll by adding it to the roll. So even expertise has an addition in there when it is applied to a roll. So, both features are doubling (multiplying) the prof bonus and then adding it to a roll. So this would violate both addition and multiplication sections of that rule. \$\endgroup\$ – Rubiksmoose Nov 19 '18 at 17:24
  • \$\begingroup\$ @Stackstuck I've updated my answer, does that make things clearer? \$\endgroup\$ – Rubiksmoose Nov 19 '18 at 17:39

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