German citizenship law is based on the principle of avoiding multiple citizenships. As a consquence anyone applying for German citizenship will generally be required to give up their other citizenship(s) and German citizens who apply for a foreign citizenship will automatically lose their German citizenship.
There are some exceptions to this principle:
While the German rules on citizenship are based on the principle of avoiding dual citizenship, this principle does not apply to children who receive dual citizenship through descent from their parents (e.g. a German father and a Filipina mother or German mother and Filipino father). In the case that the parents are not married during the time of the birth of the child that in these cases it is mandatory after German law that a formal and valid recognition of paternity which includes the consent of the mother was notarized. For more information please refer to family matters.
The German law on citizenship mandates that German citizens who voluntarily apply for and accept a foreign citizenship will automatically lose the German citizenship if they have not been granted a permission to retain the German citizenship prior to applying for a foreign citizenship.
This Beibehaltungsgenehmigung is granted by the Federal Office of Administration in Germany on an individual basis. To obtain the permission, you must prove that you still have substantial ties to Germany and are in a personal situation in which the dual citizenship would be beneficial to you and/or avoid individual detriments.
Please note that applications have to be submitted to the German Embassy and that proficiency in German is mandatory.
Further informationen can be found here Beibehaltung.
Anyone applying for German citizenship will generally be required to give up their other citizenship(s). There are exceptions to this rule if the applicant is a citizens of another EU-member state or Switzerland or if the applicant has a German parent as well as in cases of restitoration of German citizenship according to art. 116 of the Basic Law.
More Information on naturalization can be found here.