Most toilet problems aren't difficult to fix, and double-flushing is easier than most. Whereas problems like partial flushing and poor drainage can often be traced to too little water in the tank, a toilet double-flushes when there is too much. When you flush the toilet and enough water enters the bowl from the tank, suction in the pipes empties the bowl. If water keeps coming into the bowl, it fills again and suction again empties it. To prevent this, the shut-off valve in the tank has to close sooner so there is less water in the tank.
Open the tank and inspect the water level. To prevent double-flushing, you should reduce this level by an inch or more.
Unscrew the ball on the end of the float arm to make the arm longer, if you have a valve with a ball-float. This will put more pressure on the water valve and cause it to close sooner.
Lift the flapper momentarily by pulling the chain so that the tank partially drains. Allow the tank to refill. Unscrew the ball more if the level hasn't decreased by an inch when the valve shuts off.
Flush the toilet when you have succeeded in reducing the water level to an inch below its previous height. If the toilet continues to double-flush, lengthen the float arm even more by unscrewing the ball.
Adjust the float control on the vertical rod that extends through the float if your valve doesn't have a float arm. On many valves, the float fits around the valve tube, and it shuts off the valve by pushing up on a vertical rod that in turn exerts pressure on a lever to shut off the water. Pull the control, which is a plastic stop on the vertical rod, downward.
Low-consumption toilets introduced just after the 1992 law that mandated their use have had frequent problems with overfilling and double-flushing as they age. If you have one of these, it may be time to replace it.