Does proficiency with tools add to a check with a skill you are already proficient in?

Question

Just to be clear, in PHB in tools chapter is stated that:

"Proficiency with a tool allows you to add your proficiency bonus to any ability check you make using that tool."

General rule says that proficiency bonus cannot be added twice in the same skill:

"Your proficiency bonus can’t be added to a single die roll or other number more than once."

and especially this:

"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."

So, if I have 1st level Rogue with (let's say) stealth proficiency and thieves tools. Does he get bonus +4 when using thieves tools or not?

If not, what is the purpose of such tools?

Answer 1

Skill proficiency and tool proficiency are different things.

You only add your proficiency bonus to a check once. When you take an action that requires a check, if you're proficient in that kind of check, you get to add your proficiency bonus. For example, if you have proficiency in Stealth checks, then you can add your proficiency bonus to any Dexterity (Stealth) rolls you make. If you're proficient with your thieves tools, then you get to add your proficiency bonus when you're trying to pick a lock.

It seems like you're reading the rules on how to apply proficiency bonuses in a way that's more like how D&D 3.5 did it, rather than the way that 5e works. When you pick a lock, you're not making a Stealth check with your tools, you're just making a check with your tools. There is no "skill" involved when you use tools. The only source for a proficiency bonus when using tools is if you are proficient with that tool.

In your example, the Rogue would get a +2 when using the tools. If the Rogue somehow had Stealth proficiency but not thieves tools proficiency, then they wouldn't get any proficiency bonus when using thieves tools.