Third tone sandhi: when (if ever) does 3 3 become 3 0 instead of 2 3? All of the books and materials I've seen talk about the tone sandhi that changes two third tones to a second and a third, as in "你好."

I had a question about the word:


  寶寶


Some materials I've seen list this as "bao3 bao3". Based on the sandhi, I'd assume that this would be pronounced as "bao2 bao3." However, some materials have the word's pronunciation as "bao3 bao0" instead.

姐姐 is the only other word I remember seeing with this characteristic.

Is this a separate sandhi that says that a reduplicated character is always normal tone, neutral tone, that overrides the "3 3" sandhi? Is it a weird exception I just have to memorize? Or is there something else I'm missing?
"bao3 bao0" is actually not a third tone sandhi here. 

It is because the tone of second bao3 got neutralized.   

Usually a second character of a two-character word would get neutralized, I would say it is a accent thing that depends on people from different places of China.