You are a German and Namibian citizen? As a dual national you have always travelled with your German and your Namibian passport?
It is no longer necessary for dual citizens to obtain the so-called “declaratory visa”. With the letter from “Office of the Ombudsman” dated 11 March 2015, the Namibian authorities confirmed the non-requirement of the declaratory visa. You can find the document here:
Legal opinion by the Office of the Ombudsman PDF / 313 KB
Due to positive feedback from travellers in recent months, the German Embassy Windhoek will therefore no longer issue declaratory visas with effect to 1 January 2018.