Q:

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?

It normally doesn't make a huge difference, but I just got the King expansion, which gives major bonuses for those people who build the largest city and the longest road, and this slight difference in scoring may change the outcome of the game. 
The situation is similar when it comes to cities that use the same tile twice. In that case though, it seems to me that you wouldn't count the tile twice; the size of the city can be measured as the number of tiles it spans. 
However, when it comes to the road, it's not about size, but about length. If you score a point for every tile that you go through, and your road is long enough to circle back to a tile you already scored, do you score it again? 
What are the "official" rules about this?

A:

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.

