You don’t need to wait for 10 years to know if you have Langsat (Lansones), Duku or Longkong if you planted a Lansium species by seed. Mature Duku trees don’t have the bitter taste in the leaves so you need to compare the leaves of Duku and Langsat to know the difference. Sometimes even fruit experts have a hard time identifying Lansium species especially when they are looking for superior species or selecting a superior species of Langsat. In my experience, Longkong and Langsat tend to grow vertically, while Duku grows horizontally and tends to have a wide canopy if not pruned properly. If you really want to know what Lansium species you have, try tasting their leaves. Langsat has a bitter taste on the leaves that’s why also when you accidentally bite on the seeds you get the bitter taste. Sometimes we mistaken Langsat and Longkong for the same because of leaf similarities but this how we differentiate between them. On the other hand, Longkong has no bitter taste in the leaves, try eating the undeveloped seeds and small seeds, you could eat Longkong even the seeds that’s why people say its seedless because even the seeds could be eaten because of no bitter tastes but often Longkong fruit have no seeds at all and only have undeveloped seeds in it so chances of getting a big seeded Longkong is like 1:15 ratio. I already have done it, and it’s funny because I am not accustomed to eating a Lansium with seeds in it because we always eat Lansones and spit the seeds.
Also there is a way to avoid the gummy latex in Langsats. When you squeeze them properly, the tendency is that you’re going to have less gummy latex in your hands but if you're an erratic eater of Langsat then you will need to wash your hands after eating to remove the gummy latex - alcohol should do the trick!
Warning: Never eat a kilo or 2 of Lansones or any of the Lansium species without eating anything. It is the custom here to eat Lansones after meals for dessert purposes but for a snack, eat 2-5 fruits only or regret it. It’s too acidic to eat Lansones on an empty stomach.
Duku has shallow, thin, crenate, smooth leaves. No bitter taste in leaves of mature trees. Horizontal canopy.
Langsat (Lansones) has shallow, thick, acute, hairy leaves. Bitter taste on leaves & seed. Tree grows vertically.
Longkong has deep, thick, crenate and smooth leaves. Leaves not bitter. Tree grows vertically.