One of the warlock's Eldritch Invocations, Beguilling Influence, bugs me. The full description is:
Beguiling Influence
You gain proficiency in the Deception and Persuasion skills.
This is fine for a warlock who doesn't already have proficiency with those two skills, but it's a bit weak for someone who already has one of them, and entirely useless for someone who already has both of them.
I was wondering whether it might help to get some more mileage out of this invocation if I were to change it to this:
Beguiling Influence
You gain proficiency in the Deception and Persuasion skills. If you are already proficient in either of those skills, you instead gain expertise with that skill, which means your proficiency bonus is doubled for any ability check you make with it, unless it is already benefiting from a feature, such as Expertise, that doubles your proficiency bonus.
The intention is that it wouldn't be any different for a warlock who wasn't already proficient, but for a character who was already quite deceptive and/or persuasive, this invocation would enhance their abilities even further. This, to my mind, fits with what I believe is the implied flavour of this invocation; that you are being supernaturally enhanced by your patron to be more deceptive/persuasive.
I added the clause about it not stacking with Expertise (wording borrowed from the Prodigy feat) so that it couldn't be exploited by a warlock/bard or some other combination.
My question is, does this seem like a good, clean change that would add to the game, or are there issues that would make this problematic, either 1) in terms of balance or 2) going against the design of what invocations are for, or 3) that would make it far less impactful than I'm expecting?
Best Answer
I can't see this imbalancing your game, but be cautious of favoritism/unfairness and stepping on the toes of Bard/Rogue
There are a great many features that grant proficiencies with no exceptions made for people who are already proficient in those things; as such, I would be cautious of making a change to this one Warlock feature and not the various other features as well.
Something else to keep in mind is that Expertise is mostly unique to Rogues and Bards so giving it to other classes might be stepping on their toes even more.
Also note that the Unearthed Arcana version of the Squat Nimbleness feat granted doubled proficiency if you were already proficient, but in its final release in Xanathar's Guide to Everything this was removed and replaced with advantage on checks to escape grapples. That doesn't necessarily mean the doubled proficiency was bad but for some reason they did remove it.
Proficiency-granting features do a lot of different things
This is more just on overview of the various options available and their rarities.
Most features just grant proficiencies with no exceptions
I went through a list and the majority of proficiency-granting features do not have exceptions for those already proficient. This includes most of the feats that grant any sort of proficiency as well. Below are the few exceptions I was able to find:
A few features besides Expertise double your proficiency bonus:
The Scout Rogue's Survivalist feature grants them two skills proficiencies and a doubled proficiency bonus with those skills.
The Knowledge Cleric's Blessings of Knowledge feature grants them two skill proficiencies and a doubled proficiency bonus with those skills.
The Purple Dragon Knight Fighter's Royal Envoy feature grants them proficiency in Persuasion. If they are already proficient in Persuasion they can pick a different skill. Regardless, they get a doubled proficiency bonus with Persuasion.
The Prodigy feat is the only feat that lets you get doubled proficiency bonus on a skill.
A few features actually grant different proficiencies if you already proficient
The Artificer subclasses grant a tool proficiency, but if are already proficient in that tool you can pick a different one.
The Samurai Fighter's Elegant Courtier feature grants proficiency in Wisdom saving throws, but you can pick a different one if you already have that.
The Gloom Stalker Ranger's Iron Mind feature grants them proficiency in Wisdom saving throws, but you can pick a different one if you already have that.
Somewhat related, a few features that grant darkvision extend your darkvision if you already have it. Most features that grant darkvision do no such thing.
Similarly, a select few features that grant spells give you different ones if you already know them. Most features that grant spells do no such thing.
What I do at my own tables
When people end up acquiring a feature that grants them a proficiency they already have, I allow them to pick a different but thematically similar proficiency instead. This has not caused problems at my own tables, and means people don't have to worry about exactly what features are granting them what proficiencies. The main reason I do this is because I believe getting a useless feature is unfun and arguably bad design. The reason I do not give them doubled proficiency (expertise) is because I believe these should be left to the Rogue and Bard classes; skills are a lot of their entire thing/kit.