If I am building a road and I use the same tile twice, do I count that tile twice in order to determine the length of the road?
No, just count them once.

From the rules at the bottom of page 2:


  The player who has a thief on a completed road scores one
  point for each tile in the completed road (count the
  number of tiles; separate segments on a tile count just
  once).


Also the Carcassonne Annotated Rules confirms that the King expansion uses the same definition for "longest"; IE, count the tiles.