Dobaara is nothing but Hindi/ Urdu for 'again', or 'once again'.
Hence, the names Once Upon a Time in Mumbaai Dobaara, and Once Upon a Time in Mumbaai Again, are the same thing, though it appears that the makers of the movie are using the names interchangeably.
Hence, the names Once Upon a Time in Mumbaai Dobaara, and Once Upon a Time in Mumbaai Again, are the same thing, though it appears that the makers of the movie are using the names interchangeably.