How can I clean utensils (e.g., knives) used to cut longkongs?
The stickiness is similarly stubborn as trying to clean up pine sap, where regular soap often will not clean it up. 

Instead, try using mineral oil to clean up. You might have mineral oil in the kitchen for oiling cutting boards. ("Baby oil" is a form of mineral oil that also has fragrances added.) Simply rub the sticky utensils with a towel and mineral oil until the stickiness is gone.

After cleaning the sticky longdong sap off your knives with mineral oil, you will then be able to then wash off the mineral oil with soap and water.