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Doesn't look like a cocktail grapefruit to me. It seems to have a navel on the bottom of the fruits. Can you take a photo of the fruit looking from the bottom angle. If the fruits have navel, then most likely it will be one of the navel oranges. Also, the fruit flesh would be much more orange color than a cocktail grapefruit.It could be a seedling tree?
Navel is usually on the top of fruits