Ovi will be devasted and distraught over 'her' Arjun being with another woman. Savita, in a bid to hurt Archana and make her selfish granddaughter happy will spew a plan in vi's ears like so.
On the day of the wedding they will get Purvi kidnapped, or probably lock her in a room or something (who knows maybe with the help of Soham?). In Purvi's place Ovi will dress up as the Dulhan and take the pheras, get the sindoor and mangalsutra. Come time to show her face, BAM. It's Ovi not Purvi. Everyone will be shocked and hurt and upset, Arjun espeacially. But the marriage will still be real and so he will have to accept it.
Thus this is how I predict the breakup of Purvi and Arun. Nothing else makes sense. Your thoughts?