At any given time only one plane is supposed to have a certain registration. If you´ve seen a picture from the 70s, this would mean that e.g. the old airplane has been scrapped or removed from service, or sold abroad, so that the alloted registration number became free again.
I know of only one case concerning a FAA issued registration number, where two planes used te same number (illegaly). This was a guy in Germany, who didn´t want to bother with German regulations (maintenance checks, insurance) and tax, and therefore painted a fake N-number on his single engined plane, figuring that since he was operating the plane exclusively in Europe, the FAA wouldn´t catch him and the German authorities, seeing the American number, wouldn´t bother with him. His downfall came after a visiting FAA inspector noticed the number and remembered that he used to be working on a police helicopter in some American city with exactly the same number. He notified both the FAA and the German LBA
, and from then on the German pilot found himself in deep doodoo. His plane got confiscated, I don´t know if they jailed him or just fiined him, and if he ever sets his foot into the US, he´ll face legal action.