Apart from her usual 'By God'...'Sadu' lingos, her heart is exactly the same and she knows she can feel something when Mayank and Gunjan are around her.
She FIGHTS with Mayank because its in her nature to. She MELTS in front of Gunjan because its in her nature to. Today she was arguing with Mayank like she had a right to behave this way and Gunjan's touch was really familiar to her. She came back for this strange woman who claims to be her sister because apparently she wouldn't ever leave her crying. But why would that bother this new Nupur so much? So what if someone's sister would have reacted a certain way, why did she have to do the same thing? Mayank knew Nupur would rebel like this because that is how HIS Nupur would react.
I'm guessing Nupur has taken the identity of the other Nupur who died? So she just assumes this is who she has been forever.