This is an exceptional case. I have not seen such a fusion of two ovules to form a single fruit in mango. It will be good to have any further information on this.
Is this a common occurrence in your tree or orchard?
Sir, it may be because it contains multiple zygotes. It is a very rare case of this kind. But the interesting part which many people believed is that out of two seedlings germinated from such case one will be genetic clones of the mother tree. If You Arvind Kumar can try growing untill it fruit will be better.
Some Polyembryonic mango varieties, at times have misshapened fruits;
PE could be one of the probable cause, if due to genetics;
Alternately, during fruit development, any abnormal changes in hormonal synthesismay also lead to such kind of fruits, even in case of monoembryonic varieties.;