Lansium domesticum is a highly variable species, with different forms that have been classified by some taxonomists as distinct species.  There are five basic varieties, but there may be intermediate forms with overlapping characteristics:

Langsat - Slender tree with open crown, fruits in clusters of 6-40.  Ovoid fruit about 1 inch (2.5 cm) in diameter, thin peel containing latex, sour to sweet.  Contains 1-3 green, bitter seeds.  Adapted to climates with distinct seasons.  Cultivars include 'Conception' and 'Paete' (Philippines), 'Uttaradit' (Thailand), and 'LA8' and 'LA9' (Malaysia).  'Casto' is a selection from Puerto Rico with a large fruit, low latex, and high yields.

Duku - A vigorous, spreading, symmetrical tree with dense foliage, fruit in clusters of 4-12.  Fruits are spheroid, 1-2 inches (2.5-5 cm) in diameter, with a thick peel, little to no latex, and a sweet, aromatic pulp.  The seed is ellipsoid in shape.  Adapted to tropical conditions.  'Du 1' is a Malaysian variety.

Duku Langsat - Has characteristics intermediate to langsat and duku.  Fruit in clusters of 5-25, ovoid shape, sweet and aromatic, thick peel.  Varieties include 'DL1' and 'DL2' from Malaysia.

Longkong (dokong) - From Thailand, fruit spheroid, about 1.5 inches (4 cm) in diameter, thick peel with no latex, soft, sweet, aromatic pulp, and seedless or with very few seeds.

Kokosan - From Indonesia, with hairy leaves and compact fruit clusters.  Fruits dark yellow, with sour pulp and large seeds.

Source : http://www.montosogardens.com/lansium_domesticum.htm