Hey Heroico-
Due to the way that the Unreal Build Tool processes Unreal classes, it does not differentiate between the names of UPROPERTYs and the names of parameters of UFUNCTIONs. If the same name is used for a variable and a function parameter, the compiler will think that you are declaring the same variable twice. This is an unfortunate limitation of Unreal classes.
If either the variable or the parameter is not defined as being a UPROPERTY or part of a UFUNCTION, then this should not be a problem.