Can a route be included in more than one destination ticket?
Yes. The section about scoring says:


  Players should then reveal all their Destination Tickets and add (or subtract) the value of their Destination Tickets still in hand, based on whether
  they successfully (or not) connected those cities together.


So the only thing that matters is whether those cities are connected to each other. It's very possible that all your routes will share a single link across the middle of the country, and that's fine.