Did you ever find a reason why the birth was registered twice? I have just come across the same phenomenon with my grandmother. I found the GRO index confirming the registration in 1949. However, when I ordered a copy of her birth certificate, it said that it was registered in 1972. I have also found some odd GRO indexes hinting at other events in 1972 and at an earlier date.
If you can help with anything you found out I would be grateful.