A twin 76 mm battery would give Type 31 stronger all-round gun coverage

Would two automatic 76 mm guns be useful on a modern destroyer or frigate?

Yes; Type 31 would benefit from twin 76 mm guns and stronger secondary mounts, though its actual fit still provides useful coverage.

A preferred Type 31 fit would use two 76 mm weapons, or at least a forward pair combined with heavier 57 mm secondaries. The installed single 57 mm and two 40 mm guns still permit broadside coverage, but offer less individual reach and effect.