Two left joins not working
SELECT speler1.Roepnaam as team1,     -- unique alias for 1st value from 1st table copy
       speler2.Roepnaam as team2      -- unique alias for 2nd value from 2nd table copy
FROM wedstrijd 
LEFT JOIN speler as speler1           -- unique alias for 1st table copy
ON wedstrijd.Speler1ID = speler1.ID   -- join this 1st copy by its alias
LEFT JOIN speler as speler2           -- unique alias for 2nd table copy
ON wedstrijd.Speler2ID = speler2.ID   -- join this 2nd copy by its alias