This is because he did one draft by himself (granting him the first credit) and a second draft collaborating with Robert Mark Kamen (granting him the second credit).
Kamen is a frequent collaborator of French writer, director and producer Luc Besson—his co-creator on The Fifth Element, The Transporter, and Taken.
Mark Twain | Robert Louis Stevenson | Robert De Niro | Robert E. Lee | Mark | Robert Mugabe | Robert Redford | Robert Burns | Robert Bosch GmbH | Robert | Robert A. Heinlein | Robert Schumann | Robert Browning | Robert Rauschenberg | Robert Plant | Mark Wahlberg | Robert Altman | Robert Mitchum | Robert Frost | Robert Southey | Robert F. Kennedy | Robert Maxwell | Robert Graves | Mark Knopfler | Robert E. Howard | Mark Zuckerberg | Mark Rothko | Mark Antony | Robert Fripp | Robert Fisk |