Naturally Speaking

Few things in nature are as uncommon as a double banana.  The cause is unclear but someplace along the flower and subsequent fruit development, one became two.  The two were only the fleshy, edible inner parts of this seedless fruit.
To allow for the extra inner makings, the accommodating rind grew larger, too, but otherwise everything seemed normal, at least as normal as a seedless fruit can be.
Because this double banana was one finger on a hand of eight, and because commercial bananas are vegetatively propagated, there is little likelihood the trait will be passed on by this plant's progeny, wherever they may be growing.
One is left to wonder the reason and because no sexual coupling of the two gametes occurred, and of their genes, the old answer of a genetic mistake is not likely to satisfy even the most non-green thumbed banana grower or morning banana eater.