Replace the fill valve so it will fill faster. The water level in the bowl is predetermined by the design of the bowl. If it is lower that the design level, it will often take two flushes to empty properly.
The other possibility is that the water service valve is bad or the pipe that serves the water service valve may be bad. So, if the fill valve does not take care of it check out the water service valve and at the same time the supply pipe to the valve. Good Luck, Bill The Local Plumber Tustin, California http://www.TheLocalPlumber.com