Ousmane Dembele scored his first ever Ligue 1 brace as Rennes hammered Brittany rivals Nantes 4-1 despite being reduced to 10 men.The 18-year-old netted in the first and last minute of the first half as Rennes ran riot.Kamil Grosicki kept himself onside to score past Remy Riou before Cheikh M'Bengue got the faintest of touches to Dembele's free-kick, ensuring a commanding interval advantage for the hosts.Nantes' Adryan scored from close range on the hour to offer up some hope for his side but, despite having Giovani Sio dismissed for a second yellow, Rennes saw the game out.