When you get (pick up/ brew/ buy) a (unupgraded) potion that you already have without an empty potion slot, the new potion disappears for an upgrade of the existing similar potion.
Example: You have a Fire potion, a Swift potion and a Fire potion+. Now you try to pick up another Fire Potion. The first Fire Potion gets upgraded.
When you get (pick up/ brew/ buy) a (unupgraded) potion that you already have without an empty potion slot, the new potion disappears for an upgrade of the existing similar potion.
Example: You have a Fire potion, a Swift potion and a Fire potion+. Now you try to pick up another Fire Potion. The first Fire Potion gets upgraded.