Simplify the following text for a Grade 6 native English-speaking person.
Original: Much of it involved a famous Boer spy , Capt. Fritz Joubert Duquesne , who became a German spy in both World Wars and claimed to have killed Field Marshal Kitchener while en route to meet with the Russians .
Simplified: He spied on a famous Boer spy , Capt. Fritz Joubert Duquesne , who became a German spy in both World Wars and claimed to have killed Field Marshal Kitchener .