No civil war yet.

There have been altercations between Hindus and Muslims. There’s no reason to believe it would be significantly worse if you doubled the number of Muslims in India, especially when you consider that a lot of the Hindu-Muslim divisions were sown as a result of partition itself, which was an incredibly divisive and bloody affair.

Further, a larger Muslim population would also have reduced political power of the Hindu fascists as well and lack of funding through Pakistan and increased Democratic power would have led to less political disaffection in the extremist Muslim side.