Because the woman in the black cloak is locking duplicate in the room, the saying she used earlier bumping on the head and asking for another bump etc is all her telling the duplicate to convince Ranaji she is Sulakshana, the one giving the inside information how she was with Ranaji when she was there earlier. The real Sulakshana is alive but after her motor accident is disfigured and she is vain enough to want to be remembered as she was, so she hatched up this plot with Kuvar Chacha and Badi Rani Ma to cause the Gayatri and Indra split.
I think the same way Kuvar, his son and wife are terrorising Swarnalekha when Sulakshana appeared at the palace Kuvar worked on her, manipulated her into believing the best way for her was to remain hidden but as Ranaji loved her he will always be hers, because they didn't want Sona and Ranaji to be reunited and an heir would be born, now they are acting so the same doesn't happen for Gayatri as well, spoil the Gayatri relation with all this tamasha.
Sulakshana being convinced to allow Ranaji to be living in pain all this time just because of her superficial wounds, I reckon if they show the duplicate repeatedly showing things that old Sona would do to convince Ranaji then we can be sure that the cloaked lady is definitely the real one feeding her information. Definitely duplicate is on Sona because she wasn't hurt at all with her motor accident, them also after the car fell off the cliff they didn't search the cliff side to look for any clue if she threw herself out of the car? 😆