Why are there double paths?
It depends on the number of players. The second path is available when you have four or more players.


  Important Note: In 2 or 3 player games, only one of the Double-Routes
  can be used. A player can claim either of the two routes between cities, but
  the other route is then closed to other players.


Note that no single player may take both routes of a Double-Route.


  Some cities are connected by Double-Routes. One player cannot claim
  both routes to the same cities.