Dual citizenship is allowed in a bunch of countries to maintain so called 'unity within the family'. This is to prevent children from having to choose the nationality of one parent over the other - and thus allows them to live/work in the country of either as adults.

The same often happens in the case of spouses - who do not have to give up their original citizenship once they become naturalized citizens of another country - so as to prevent the situation that their children would have the mothers/fathers citizenship (thru birth) while the mother/father has to give it up while taken up citizenship fo the spouse.

In my experience - laws and procudeures concerning citizenship are often opaque and contradictory - often on purpose so as to discourage dual citizenship (while a right under the constitution or treaty). If you fight for it and jump thru all the hoops - you will often be succesful - even if initialy bureaucrats state it is not possible or present you with a catch-22 situation.