Years ago a certain flight line had two aircraft with sequential rego, lets call them BBA and BBB. A new PPL holder lodged a flt pln for a xcountry under BBA, took off in BBA and proceeded to make all calls quite normally and correctly; but as BBB. Tower probably recognised this was happening but didn't quite know how to acknowledge the calling a/c, BBA by the flight plan or BBB as it was being broadcast. In the end they had to ask,
"BBA, are you BBA or are you BBB?" No answer was the stern reply.