Langsat and duku. These are related (different groups of cultivars of the same species), and vary slightly in appearance and taste (though not so much that I would be able to tell you what the differences are). These were labeled as “dukong” which is supposed to be a cross between langsat and duku. Similar to a lychee, there is a peelable thin outer skin which contains the inner tender flesh. The flesh divides into segments (like a mangosteen) and some larger segments contain a single seed. The fleshy segments are very juicy, and have a flavor like a sweet, mild grapefruit (with a tartness to it instead of the bitterness a grapefruit would have).
There’s a wikipedia article if you want more information at https://en.wikipedia.org/wiki/Lansium_parasiticum