This is by no means a contradiction.
The reason why alcohol is not permitted in this world is due it's harmful and evil affects. It causes the person to be robbed of their mental faculties and causes numerous harmful affects on the body, as we all know.
In Jannat however, wine will not have these bad qualities as it does in the dunya. Allaah (subhaanahu wa ta'alaa) mentions:
"..A cup from a gushing spring is brought round for them, White, delicious to the drinkers, Wherein there is no headache nor are they made mad thereby..." (37:45-47)
Maybe someone can further elaborate inshaAllaah.
Wallaahu A'lam.