A creature can move twice on its turn if it takes another move action instead of a standard action. Taking the same move action twice in a row is called taking a double move.
Same Move Action Twice: To take a double move, a creature must take the same move action twice in a row on the same turn—two walks, two runs, two shifts, or two crawls.
Combined Speed: During a double move, first add the speeds of the two move actions together. The creature moves using the combined speed.
Because of this greater speed, the creature can sometimes move into a square that it would otherwise be unable to enter.
Example: A creature whose speed is 5 can enter only 2 squares of difficult terrain (see below) when it takes a single move action to walk. If it takes a double move by walking twice in a row, it can move a total of 10 squares, so it can enter 5 squares of difficult terrain instead of only 4.
Occupied Squares: During a double move, a creature’s first move action can end in an ally’s space, since the creature is not stopping. The second move action follows the normal rules for where the creature can end the move.
Published in Player's Handbook, page(s) 284, Rules Compendium, page(s) 205.