I should also add that the most bizarre case of dual citizenship I have come across is with the USA.
If you are a US citizen and marry an overseas citizen (say Thai) and your children hold dual citizenship (US and Thai), if the child does not reside in the USA for a period of 3 years (I think, could be longer), then any children that your child has [your grandchild] will not be entitled to US citizenship.
I know this is true because i could not apply for US citizenship based on the fact that my grand father (and Aunt for that matter) was a US citizen.
Not sure if this applies elsewhere, but had me thinking.