Wouldn’t be the first time - but I can swear to you I went through the process. I had all the paperwork for the German citizenship - and was told I needed to go get the final paperwork signed off by the US Embassy. I schlepped over there and that is where they told me this was not going to happen unless I gave up my US citizenship. They specifically told me it was one thing for a German to get a US citizenship (and they could keep their German citizenship), but it was not possible for a US citizen to get a German citizenship and keep their US passport! I can assure you that this not only happened to me, it happened to my good friend Michael (born in the US but raised in Germany - and he only had until he was 18 to decide which country - thus his sister was American and he was German) and also my American friend Ric who also thought this would be a good idea, but got the same song and dance from the US Embassy when he had gone there a couple years before I did this (we didn’t compare notes until later). Maybe it was just the US Berlin Consulate that was ill-informed.
Sorry to dwell on this and hijack my own thread, but it does piss me off to know I could have completed all of this ages ago with no problem - and am only just now finding out. I did get a permanent residency/work permit (good for life) back then, so it was/is no huge deal for living there - but still.