That because when you place BlueprintNativeEvent, UHT will automatically generate deceleration of _Implementation function by it self, thats why you getting “already declared” error. It’s doing that because 1. It needs that function to be declared for compiler to build generated code 2. So you don’t need to declare it when you don’t need it, other wise you would get undenefied function errors, forcing you to declare it . Just place function definition in cpp file and thats it.
Remember that UHT generated code for you function and paste via GENERATED_BODY() macro