Harvesting poisons when I have proficiency in both Nature and Poisoner's Kit

[Expected]

If I am attempting to harvest poison and I have proficiency in both Nature and the Poisoner's Kit, does the DM choose which I apply to my check or do I get to add twice the proficiency bonus (similar to Expertise)? If I don't apply both proficiencies to the check, is it even worthwhile to have proficiency in both Nature and the Poisoner's Kit?

[JoeJ]

If I am attempting to harvest poison and I have proficiency in both Nature and the Poisoner's Kit, does the DM choose which I apply to my check or do I get to add twice the proficiency bonus (similar to Expertise)? If I don't apply both proficiencies to the check, is it even worthwhile to have proficiency in both Nature and the Poisoner's Kit?

I'm not sure what you mean by deciding which one; your proficiency bonus is the same for everything you're proficient in. You never add your proficiency bonus to a roll more than once*. If you have both proficiencies, however, the DM might consider letting you roll with advantage.

* There are a few circumstances, such as having expertise in something, when your bonus is multiplied before being added, but it's still only added once. Your bonus is only multiplied this way when some rule explicitly says it is.

[Ventruenox]

Per Xanathar's rules, you would be able to apply both bonuses to a Nature check to figure out which creatures could have harvestable venoms, or other knowledge style ability check. When it comes to harvesting, however, you only apply the Poisoner's Kit proficiency bonus if you don't have Nature proficiency (DMG pg 258). At best, you get to pick the higher of the two.