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In Summary, you can’t really tell unless you use genetic markers.
For some Polyembryonic mango varieties, the dominant seedling is the clone but in other varieties, the weaker seedling is the clone.
If you work with a specific variety for a long time you may become familiar with it and be able to take educated guesses.
If you have a Polyembryonic seed and multiple sprouts come up, I would recommend you grow out at least two of the seedlings and there will be a chance you will get a clone.
A Polyembryonic seed can have more than one zygotic seedling come up and Monoembryonic variety seeds can have multiple sprouts but usually with one origin for the root so it is a very complicated subject.
Simon