Din färdighet fördubblas bara en gång

På 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.

Generellt stackar inte saker eftersom PHB specifikt anger utgångspunkten:

Dragon Ancestor (PHB s.102, betoning min):

At 1st level, you choose one type of dragon as your ancestor. The damage type associated with each dragon is used by features you gain later.

You can speak, read, and write Draconic. Additionally, whenever you make a charisma check when interacting with dragons, your proficiency bonus is doubled if it applies to the check.

Kompetens (PHB s.54, betona min):

At 3rd level, choose two of your skill proficiencies. Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.

At 10th level, you can choose another two skill proficiencies to gain this benefit.

Ingen av dessa ändrar din kompetensbonus, bara vad som görs på rullen.
Detta betyder:

• dragonAncestorBonus = proficencyBonus × 2
• expertiseBonus = proficencyBonus × 2

Eftersom båda börjar med din kompetensbonus får du välja:

• totalt = rulla + proficencyBonus
• totalt = rulla + dragonAncestorBonus
• totalt = rulla + expertisBonus

För att göra saker tydligare, säger PHB (s173-174):

Your proficiency bonus can't be added to a single die roll or other number more than once. For example, if two differenct 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 (double 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 will add it only once and multiply or divide it only once.

För att komma runt denna begränsning, skulle förmågan behöva likna "ökar övertygelsesförmåga med 5" eller "lägger till 4 för alla övertalningskontroller".

För det mesta hittar du inte stora stackeringseffekter i D & D-5e. Om du tror att du hittat något som är trasigt eller för bra för att vara sant, är det förmodligen.