Simplify the following text for a Grade 7 native English-speaking person.
Original: There can only be one Princess Royal at a time , so Elizabeth II never held the title because her aunt , Princess Mary was already called Princess Royal .
Simplified: The style is held for life , so a princess can not be given the style during the lifetime of another Princess Royal . In particular , Queen Elizabeth II never held the title as her aunt , Princess Mary , was in possession of the title .