The 10-speed Ultegra shifters, 6600 & 6700 are double or triple specific. The double shifter will not work on a triple.

The 9-speed Ultegra shifters, 65xx are double/triple compatible.

Shimano is now making a double specific 10-speed 105 shifter. They have been replacing under warranty some 105 shifters that failed when used on a double.