There are double coconuts (coco de mer) trees growing here in Hawaii, both at botanical gardens and in private collections. It hasn't been fruited yet because not only do they take an extremely long time to fruit, but also you need both male and female plants. Add to this the difficulty of getting seeds from the Seychelles, and it becomes nearly impossible. About a decade ago the country made it illegal, without a special permit, to send the seeds out of the country.