The new (or higher level) effect takes priority.
The Pathfinder 2E General Rules state that duplicates of the same effect (such as a spell with overlapping duration) do not stack.
When you’re affected by the same thing multiple times, only one instance applies, using the higher level of the effects, or the newer effect if the two are the same level. For example, if you were using mage armor and then cast it again, you’d still benefit from only one casting of that spell. Casting a spell again on the same target might get you a better duration or effect if it were cast at a higher level the second time, but otherwise doing so gives you no advantage.
When you re-cast flame wisp, you will have 3 wisps. One and only one wisp will hit a target when you successfully Strike them while the spell is active. This can still be useful if you have already used up some of the wisps from a previous casting.