See: WHO'S WHO.
{informal} 1. Who this one is and who that one is; who the different ones in a group of people are or what their names or positions are. * /It is hard to tell who is who in the parade because everyone in the band looks alike./ * /It took the new teacher a few days to remember who was who in the class./ Compare: WHICH IS WHICH. 2. Who the important people are. * /John didn't recognize the champion on television. He doesn't know who is who in boxing./ * /After about a year, Mr. Thompson had lived in this town long enough to know who was who./ Compare: WHAT'S WHAT.