Two heads, two tails, curved spines, and other massively debilitating mutations are not at all uncommon in fish. It is nature's way. For every "good" genetic mutation (i.e. evolutionary improvement), there must be thousands of "bad" ones. This poor specimen is one of them. In nature it would have been eaten almost immediately. Even in the safety of an aquarium, I highly doubt it will live much longer. Keeping it alive by taking extraordinary steps would be cruel and unnatural.
Do the right thing.