Is there any precedent for doubling the proficiency bonus only if the character has a given skill proficiency?

Question

A number of character classes have features that allow them to double their proficiency bonus when using selected skill proficiencies. An example is the Bard's Expertise:

[C]hoose two of your skill proficiencies. Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.

As far as I can tell, there seems to be two ways to go about this:

1. Let the character select from among their existing skill proficiencies. Examples: Expertise (Rogue), Expertise (Bard), Canny (Ranger, TCoE optional feature).
2. Let the character select proficiency from a list of a specific skills and immediately grant the ability to double their proficiency bonus with the selected skill. Examples: Blessings of Knowledge (Cleric Knowledge domain), Survivalist (Rogue Scout).

The question is whether there is any precedent for requiring the character to choose from a list of specific skills where the character would not automatically get proficiency if they don't already have it, and would therefore not gain any benefit if they did not have proficiency in any skill in the list.

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Not relevant to the answer, but my motivation has to do with ideas for homebrew classes and features. In particular, I am put off by the way the Scout's Survivalist class feature works. If a player intended to play a Scout, the optimizing choice is to not take a background with Nature and/or Survival, because at 3rd level you will get both skills and expertise (https://rpg.stackexchange.com/a/125024/25766). If you already have the skill, you still get expertise, but it's a wasted skill proficiency slot.

What I would like to do is create a class feature that either:

1. Grants proficiency in a skill if the character doesn't have it, or grant expertise if the character already does have it.
2. Grants expertise if the character has the skill, but no new proficiency if they don't.

Obviously, homebrew is homebrew, but I'd like to know whether anything published does something similar.

Answer 1

No official character options do this exactly

There are no official character options that do exactly what you're proposing, but there are some character options which look similar if you squint - generally, they grant expertise if a character is already proficient but only in certain conditions.

The ranger's Natural Explorer feature upgrades proficiency to expertise when the ranger makes Intelligence or Wisdom checks about their favoured terrain:

When you make an Intelligence or Wisdom check related to your favored terrain, your proficiency bonus is doubled if you are using a skill that you’re proficient in.

A draconic bloodline sorcerer's Dragon Ancestor feature upgrades proficiency to expertise on charisma checks when dealing with dragons:

Additionally, whenever you make a Charisma check when interacting with dragons, your proficiency bonus is doubled if it applies to the check.

The artificer's Tool Expertise feature upgrades proficiency to expertise for any tool the artificer is proficient with - so not strictly affecting skills, but it's the closest to the second mechanic you proposed, in that it grants expertise on a proficiency if and only if the character already has it:

Starting at 6th level, your proficiency bonus is doubled for any ability check you make that uses your proficiency with a tool.

There does exist a class feature which is designed in such a way as to avoid the problem you have identified, though. The Purple Dragon Knight fighter's Royal Envoy feature grants persuasion proficiency and expertise but in a more forgiving way than most features, allowing the choice of a different proficiency if the character is already proficient in persuasion:

At 7th level, you gain proficiency in the Persuasion skill. If you are already proficient in it, you gain proficiency in one of the following skills of your choice: Animal Handling, Insight, Intimidation, or Performance.

Your proficiency bonus is doubled for any ability check you make that uses Persuasion. You receive this benefit regardless of the skill proficiency you gain from this feature.

Expertise in persuasion is an important feature for the Purple Dragon Knight so that is always granted, but unlike most features of this kind it takes care that the basic proficiency it would grant is not wasted if the character was already proficient.

Aside: I personally prefer to just rule that, much like character creation allows overlapping skill proficiencies to be reassigned as free choice, if a class feature would grant you proficiency in a skill you already have, you can instead freely choose a new skill proficiency. Then there's no proficiency disparity between a character optimally built at a higher level and one who levelled there naturally. (But to be clear this is in no way RAW; those class features all state that you gain a proficiency only if you don't already have it, and the rule about free-choosing proficiencies when they overlap only applies to backgrounds/character creation.)

Answer 2

There is no official instances where not having proficiency in a skill would deny access to a "double proficiency" feature

This is just speculation, but I would assume this is because such an instance would require the player to make one of 2 bad choices:

1. Not take the required proficiency, and miss out on a feature of the class, or
2. Take that specific proficiency in order to take advantage of the classes features.

In the first instance, the player is willfully removing access to a class feature, which is a very poor choice, and would likely never happen for all but the most inexperienced players. The second situation is basically forcing the player to take a certain proficiency to fully utilize the class, so why not just make that proficiency a guaranteed part of the class anyways? Whether the character gains access to one specific skill, or if it is chosen from a list, if they will only be able to gain double proficiency for specific skills, and they must already be proficient, then the player will select that skill every time.

Something you may be missing, answered here, is what to do if you gain access to a proficiency from two different sources. In your example of the Scout Rogue with a background that gives them proficiency in nature or survival (Say a Golgari Agent), the character would have proficiency in Nature and Survival from first level, and upon reaching third level would gain double proficiency in those skills, and regular proficiency in two other skills of their choice.