The duku and langsat are closely related members of the same botanical family ( meliaceae), and are very similar fruits. Both are slightly larger than a golf ball and grow in clusters with a thin, leathery skin. The duku has a golden brown color; the langsat is more of a cream color. (Brown langsats are over-ripe.) Gentle pressure on the top (stem end) of the fruit will cause the skin to split, making the fruit easy to peel. The skin of the langsat contains a sticky latex which is annoying but not harmful. The duku does not have this latex sap, and is considered a superior fruit for that reason.
The interior of each fruit contains 5 segments which are very much like the segments of a small grapefruit. The flesh is sweet and juicy, and tastes much like a very mild, sweet, grapefruit. Some segments may contain small, bitter seeds. Do not bite into them!!
We have not seen duku in the Philippines, but langsat is widely available here when it is in season, beginning in late fall.