If you are entitled to, and acquire the German citizenship, you can keep your original nationality. However this does not work the other way round. I.e. if you are a German and request a second nationality you have to waive your German nationality. There are some exceptions for example if you want to get the nationality of another EU member state, or if you have good reasons to keep both passports, which requires the prior approval from the German authorities.
Netanyahu: Most of Americans see eye to eye with Israel on dangers of Iran deal (Haaretz)
from the article: Fear is driving Israelis to obtain foreign passports