Duku, Langsat, Duku-langsat, Dokong.

http://www.fruitlovers.com/langsat2.html

DIFFERENCES BETWEEN 3 TYPES OF LANSIUM DOMESTICUM: LONGKONG (also called Dongkong), LANGSAT, and DUKU (also called Duku-Langsat)

	LONGKONG	LANGSAT	DUKU
FRUIT SKIN	Thick , rough, pale yellow color, no latex.	Thin, soft hair, yellow, there is latex when broken or torn.	Thick, soft hair, rough, pale yellow.
FRUIT	Round, big shape, clear flesh, sweet, or sweet-sour, thin seed, full flesh, good smelling.	Smaller than Longkong and Duku, 1 inch wide, 1.5 inches long, egg shaped on bottom.	Bigger than Longkong and Langsat, round shape, strong smell, non bitter tasting seed.
SEED	Few seed in one fruit. Developed seed is quite big, curve on one side, and flat on the other side. Polyembryonic.	Small seed, green color. Strong bitter taste. Two pieces with a crack line in between. Monoembryonic.	Football shaped seed. Monoembryonic

Origin

It originates from western Southeast Asia, ranging from Southern India to the Philippines. It is found easily in Malaysia and Thailand. It belongs to Meliaceae family.

Varieties of Langsat Tree

Varieties of lansium domesticum fruit in Malaysia. Below shown variety and its differential between:

Duku 
-round shape with thick pale yellow rind. It is the bigger size than langsat and dukong, sweeter than langsat. It is the biggest size comparable to Dukong and langsat.

Langsat
-egg-shaped fruits with the thin yellowish leathery cover by soft hair, there is latex when peeling off. It is rougher than duku, and green color seed. The flesh was strong bitter taste. It is smaller than dukong and duku

Dokong or Longkong
-The thick rind in pale yellowish color. There is no latex while peeling off. Dokong is a hybrid of duku and langsat with very few seeds. This is the more expensive among Duku, langsat, and Dukong.

https://www.amazon.com/Longkong-Tropical-Domesticum-Thailand-Phytosanitary/dp/B075BJQK3K

Originated from South East Asia, Duku or Langsat is an exotic, seasonal fruit in the tropics and so it is called in Malaysia and Indonesia. The scientific name is Lansium domesticum. Langsat is also known in English as Lanzones. In Thailand, the same fruit is called Longkong. And a local fruit vendor told me the fruit is Dokong!! According to the seller, this variety is sweeter, nicer owing to smaller or undeveloped seeds and thicker rail, so it is very much preferred over the standard Langsat. My wild guess, this is a cultivar between Duku and Longkong, thus gives the name Dukong or sometimes refers to Dokong!

https://mbg.com.my/blogs/news/dokong-translucent-juicy