I just googled it and it seems that its not rare, but not common either, one in 3000 women will have a second placenta, is some quite interesting info on twinning incidences and sole survivors or twinning conceptions.
There is also a thing where the placenta appears to be two but is only really double lobed and is connected by vessels and arteries etc.
My sister quite a few years ago now told me she had this happen with 1 of her pregnancies, but that it was origionally twins... but when it comes to my sister, I never know what to believe when it comes from her mouth so I don't know how true she was being IYKWIM!... I haddn't heard of it before then so I didn't really give it another thought.... until now.
With DS, I had an extra bit of placenta. It was like a smaller placenta sitting on the normal placenta. It wasn't noticed until DS was born. The ob sent it off for testing but it all came back normal, and they said it was just an extra bit and that was that.
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