I don't know what is wrong with so many negative reviews... it's actually a beautifully scripted movie for the average desi audience.
The concept was adapted from one of my all-time favourite Hollywood movies - "Frequency" (and later, the Spanish movie, "Mirage")
But I honestly did admire how it was adapted in the Bollywood style. And Taapsee being in it made it more watchable 'coz I'm quite a fan :)
Go slow, keep an open mind and make sense of it... the movie does require brains - it's not just a random entertainment/masala film.
And yes, if you question it from a quantum relativity point of view, theoretically, there are numerous possibilities of an experience like this occurring in the real world.
The beauty of this movie is that it practically engrosses you with the plot by simply plucking a hypothesis from a plausibility in real-world space-time physics.
Enjoy if you're a nerd and an even bigger Bollywood fan :)