Hi RoadStar
Without seeing the error you are asking about, its hard to understand what you want as an answer 
I read your question two different ways so here are two answers
- Read as “why do I see this twice?”
the .h file contains the definition (the syntax of the function/method)
the .cpp file contains the implementation (the logic that actually does something)
- Read as “There is some sort of error about redefinition, but you didnt paste the error”
If you want to make your own version of a file in the parent the parent has to have first created it as “virtual”, then you can override it
//In .h file
bool IsActorSelected(ATriggerBox* actor) override;
Hopefully one of them answers your question.
NB. using CODE ] tags is easier to work with than screenshots of your code.