This is not quite right. I always depends how you obtained your other non-German citizenship.

Example one)
If you are born to German parents in a country where you get the citizenship by being born there it is not a problem. The classic is example is being born in the United States of America to German parents. This is tolerated. I think it is technically a loop hole in the system but this is my personal non-professional opinion.

Example two)
You are born in Germany to parents which immigrated obtained there German citizenship but never really gave up there original citizenship. Now the child is German and has the original citizenship. Here the German law says that the child can keep both citizenships for a certain time but has to decide (I think as soon as it turns 18) which to keep. This is the classical German/Turkish dual citizenship dilemma. The same does apply if you are born to a US and German citizen in Germany. You will be an US citizen and German but technically you need to decide when you are 18.