Some sources say that aggregation means that the class owns the object and shares reference. Lets assume an example where a company class holds a list of cars but departments of that company has list of cars used by them.

class Department
{
   list<Car> listOfCars;
}

class Company
{
   list<Car> listOfCars;
   //initialization of the list
}

So in UML class diagram, I would do it like this. But I assume this is not allowed because it would imply that both company and department own the objects..

[COMPANY]<>------[CAR]
[DEPARTMENT]<>---|        //imagine this goes up to the car class