[kiuyhjk]

Some countries don't allow dual citizenship. My German friends had to renounce their German citizenship when they became Americans (again for the green card). It made it very tricky tax-wise when their parents died and they suddenly became foreigners (to Germany) owning a lot of German assets.

[sasvari]

as far as I know, it's possible to apply for keeping your German citizenship when gaining the one of another country. you have to justify it with family relations or something like that. at least it's possible, but I don't know about the trouble you have with that ...

[trevi]

Indeed. You also keep your German or Austrian citizenship if you did not apply for the other citizenship (e.g. if someone was born from German parents in Australia).

In general different countries have (very) different sets of rules, see e.g. http://en.wikipedia.org/wiki/Multiple_citizenship